There are a variety of useful ways to find the correct exposure. After you become experienced it might be as simple as observing the projected image focused onto an upside-down scrap print (upside-down for the reflective white surface) and using your experience to “guesstimate” the initial exposure. Then you'd fine-tune the exposure with test strips or single test patches,
One nice way to save time and material is through the use of a 4” x 5” Kodak Projection Print Scale. These are no longer made, but can be found cheaply on eBay used. The condition can vary from perfect to grubby. You have to ask the seller for details before buying.
https://www.ebay.com/sch/i.html?_fr...S0&_nkw=kodak+projection+print+scale&_sacat=0
Here’s how to use it. It’s quite simple and works well.
"USING THE KODAK PROJECTION PRINT SCALE
With the KODAK Projection Print Scale, you can determine the starting exposure time for black-and-white or color enlargements, or contact prints on paper or film. See the test print on the back of this sheet.
For Enlargements:
1. Place the negative or transparency in the enlarger and focus sharply.
2. Turn off the printing light and other white lights.
3. Place the enlarging paper (of film) on the easel.
4. Lay the scale (shiny side up) on the paper over the center of interest of the image; hold it flat in contact with the paper (or film) with a sheet of glass or your hands.
5. Expose for 60 seconds* and develop it as recommended by the manufacturer.
6. Inspect the image and select the sector that appears to have the corrct density (darkness). The number on the rim of the sector is the starting exposure time in seconds.
7. Without the scale, make an exposure at this time on another sheet.
Note: Most photographic prints appear slightly darker when dry. Learn to adjust for this “dry down” when judging wet prints.
For Contact Prints or Negatives:
1. Place the scale on the contact printer glass, shiny side down.
2. Place the negative or transparency, and paper, in their usual relative positions over the scale.
3. See steps 5 and 6 and determine the correct exposure.
Although the segment may appear to have the correct density, when the entire print is made it may be slightly darker or lighter overall. This is because a small segment, out of context of the rest of the picture area, is not always an exact indicator of the correct density.
*If the exposure time is expected to be short, make the test exposure 30 seconds. The correct printing time will be half the number of the best sector. For printing black-and-white negatives from color transparencies, use a 6-second test exposure, and a printing time 1/10 the number of the best density sector.
EASTMAN KODAK COMPANY, Rochester, NY 14650
9-78-AXX"
The 10 segments correspond to exposure times in seconds as: 2, 3, 4, 6, 8, 12, 16, 24, 32, and 48.
Here’s a new one sold under the Delta 1 Brand. It has only 8 density/time segments: 2, 3, 5, 8, 12, 16, 32, and 48 seconds.
https://www.freestylephoto.biz/41621-Delta-Projection-Print-Scale
https://www.bhphotovideo.com/c/product/15711-REG/Delta_12610_Projection_Print_Calculator_Scale.html