Apart from my lici colorstar 3000 (oh will I be sad when the time comes that it dies and I can't fix it) the handiest is a ring around. Then the next best tool -

The Kodak Ektacolor FilterFinder Kit. Kodak Publication No. R-30, edition 1, 1978. Old cat # was 102 7408. Mine I got used, and it has the original price of $29.40, which was a lot of coin in the late 70's.

It includes a two page ring around that also appears in other Kodak publications of the era, and a 8x10 grey card, of the same sort that Kodak sold independently for about $30 for two of them at the same time.

There is a dial calculator, much of the sort that shows up in other Kodak darkroom data guides, to help you figure new exposure times at a different head height once you have a good small print, with the exposure correct.

This one also includes corrective scales to help you find the right tiime based on your test tiem if the exposure is judged to be 1/2 stop too light, too dark etc, or the new time if you are adding in new acetate filters to correct color cast. This dial I originaly worked without by using filter factor tables and a scratch pad and pocket calculator.

The real gems of the kit are the last 2 bits - One is a peice of 4x5 transparency film, and the other is a density reference patch.

You photgraph the grey card in the light situation that the print you want to get filtration sort out for, ideally on the same roll, but the same type and speed of film, developed the same way will work lamost as well. (In a pinch without a grey card at hand, I have also used weathered asphalt road paving as a grey reference source and then slightly adjust exposure to suit later.)

You project the grey card image and print though the transparency film which is in contact with the paper on the easel. The transparency has patches of 0-30cc of color in the yellow, magenta and cyan axes. It also has places to encourage you to record what test print number was made, the filtration used to measure it, which of the test squares is closest to grey, and the exposure correction factor.

The locator card has 1/2" square patches of varying degrees from white to black with 18% grey in the center, and two stops away in either direction in half stop increments. Each sqaure has a bit smaller than 1/4" hole punched in the middle of it. They are numbered for the exposure factor: 4 (white), 3, 2, 1.5, 1 (mid grey), 0.75, 0.50, 0.38, 0.25 (balckest) This tool can be made by careful exposure of b&w photographic paper.

You use this tool to judge how dark the test print grey was made though a clear part of the transparency overlay. Say that the grey was 1/2 stop overexposed. Then you use the window in patch of .75 denisty to look for the gery square in the colour varied squares part of the exposure. This allows you to refine the color further without trying to first nail the right denisty perfectlly on the wrong filtration, and hence gets you that much closer on the next iteration.

There is also a stright forward 11 page illustrated booklet on how to use the kit best, and what to do if things are not going well.

If there is interest, I may attempt to clone the transparency as a project over my Christmas holidays. I have a box of about 37 sheets of 4x5 Astia that I consider my evenest transparency film, and may see if a dupe using pre-flashing to control contrast can yeld an acceptible clone that can be measured against the original on my denisitometer.