Old-time tool for answering these questions as they come up: a slide rule. I always kept one in the darkroom, and now keep one within reach of the dry, bright desk when I'm doing post processing and printing.

The benefit is that when you set a number on the C scale next to a number on the D scale, all the other numbers on the C & D scales are the same ratio to each other. (There are no decimal points on these scales, so 2 over 3 is the same as 20 over 30, etc.) While you don't have the third-decimal-place precision you get with a calculator, you CAN SEE and scan ALL the ratios AT ONCE.

While there are collectible slide rules that fetch high prices, ones that would be suitable for print sizing purposes are available beginning at a few bucks on ebay and probably elsewhere. I prefer the Pickett aluminum models with black numbers on yellow, just for ease of reading, but that's a personal choice. And if you can find an affordable CIRCULAR one ("Concise" brand is one I bought new within the last 10 years, but I can't find a listing for one at the moment) you never end up with the slide sticking way out of the tool.

Old-time tool for answering these questions as they come up: a slide rule. I always kept one in the darkroom, and now keep one within reach of the dry, bright desk when I'm doing post processing and printing.

The benefit is that when you set a number on the C scale next to a number on the D scale, all the other numbers on the C & D scales are the same ratio to each other. (There are no decimal points on these scales, so 2 over 3 is the same as 20 over 30, etc.) While you don't have the third-decimal-place precision you get with a calculator, you CAN SEE and scan ALL the ratios AT ONCE.

While there are collectible slide rules that fetch high prices, ones that would be suitable for print sizing purposes are available beginning at a few bucks on ebay and probably elsewhere. I prefer the Pickett aluminum models with black numbers on yellow, just for ease of reading, but that's a personal choice. And if you can find an affordable CIRCULAR one ("Concise" brand is one I bought new within the last 10 years, but I can't find a listing for one at the moment) you never end up with the slide sticking way out of the tool.