OVER the years, meeting planners have developed guidelines and formulas to help them estimate their audiovisual needs. The following are some of the most useful rules of thumb that can help you to match your groups with the meeting room and equipment that fit the job.

First Row/Last Row (2X8 rule) No one should be seated closer to the screen than two times the screen height, and no farther away than eight times the screen height. For example, if you were using a 9-foot-high, 12-foot-wide screen to show a video, the audience should not be any closer than 18 feet from the screen, and the last row should not be more than 72 feet away.

Screen Height One way to do a quick estimate for an appropriate height for a screen is to divide the length of the room by eight (assuming that the entire room will be used). For example, if a room is 60 feet long, the screen should be at least seven feet tall.

Screen Width Once you've determined the height of the screen, the width is determined by the medium. For slides, the height-to-width ratios for horizontal slides is two to three (if the screen is 8 feet tall, then the width should be 12 feet). If you have a mix of horizontal and vertical slides, the ratio is one to one. For a wide-screen format, use a one-to-three ratio (two horizontal images side by side is a common multi-image format, called "two-screen butt"). In this case, with an eight-foot-high screen, the width should be 24 feet.

For overheads, the height-to-width ratio is one to one. Video and 16mm film require a three-to-four ratio. If you transfer a slide show to video, you need to be aware that their aspect ratios are different. If you crop the video so there is no blank space above or below the slide, you will lose some of the image on the sides. If it's important to keep the entire slide image, you can use a small border above and/or below the image on the video transfer.

Minimum Ceiling Height Even when people are seated, their heads can block the view of those behind them. A quick fix is to raise the bottom of the screen so it is four to five feet off the floor. This should give even those in the rear a good view of the screen. Of course that means that the ceiling height of the meeting room must be be at least as tall as the height of the screen plus four or five feet.

Calculating Room Size To calculate the size room you need for a function, multiply the number of persons in the audience by the "comfort factor" and then by 1.5 (to account for aisles and staging space). Here are the comfort factors:

* Theater style. Comfortable: nine square feet; tight: seven square feet

* Theater style with AV. Ten square feet

* Classroom style. Comfortable: 18 square feet (two persons per six feet); tight: 12 square feet (three persons per six feet)

For example, if you have an audience of 500 people and you're using AV, multiply 500 times 10 times 1.5. The product, 7,500, is the number of square feet of space you will need. Of course, whether you're projecting from the rear of the screen or from the front has an impact on the amount of space required. The calculations above are for front-screen projection.

Another way to calculate how many people you can put in a room is to subtract the space you need for the stage, lighting towers, control station, scaffolding, etc. from the total space and then divide the remaining space by ten square feet.

Lens Focal Length How big a lens do you need to fill the screen with an image? To calculate the lens focal length that you'll need for a slide projector or 16mm movie projector, multiply the "throw distance" (in feet) by the aperture width (in inches) and divide by the screen width (in feet). The throw distance is the distance from the projector to the screen. The common aperture widths are 35mm (1.35 inches) and 16mm (.38 inch). For example, if you're projecting a slide from a distance of 18 feet to a screen that is 12 feet wide and you are using a 35mm lens, then the focal length you will need will be two inches (approximately 50mm). If you already have your lens, you can calculate the throw distance by multiplying the focal length by the screen width and dividing by the aperture width.

Guidelines for Video Projection When a group is viewing a video monitor, 25 people can watch a 20-inch monitor, 50 people a 27-inch monitor, and 70 a 35-inch monitor. When using a CRT (cathode ray tube) video projector, the projection distance is 1.5 times the screen width. So, if you have a 12-foot-wide screen, the projector would need to be 18 feet away.