A "working solution," by definition, is already diluted correctly. Use it as is. If you are mixing from the concentrate or from powder, follow the manufacturer's instructions for mixing and diluting to a working solution.

As for clip tests: yes, some film clear very quickly in fresh fixer. I know that common wisdom is to fix for twice the clearing time, however, 20 seconds of total fixing time is just too little. There are those who recommend at least three times the clearing time plus a bit of a safety factor to account for the slowing down of the fixer activity during the actual fixing process. (Do a clip test before fixing a large batch of film and then immediately after; the second clearing time will be longer.)

Add to this the fact that modern films use harder-to-fix silver compounds and complexly bound sensitizing dyes which require more time in the fixer, and you can see that using the minimum 2x-clearing time is risking under-fixing.

Film is not adversely affected by somewhat longer times like fiber-base paper since it is coated on a waterproof substrate. Longer will not hurt and ensure 1) proper fixing and 2) that all the sensitizing dyes get dissolved out (really one in the same thing. Use the manufacturer's maximum recommended fixing time as a standard. Using this time will not damage or bleach your negatives in any way. Do the clip tests to find out when your fix is exhausted.

If you are really interested in fixing for optimum permanence, look into two-bath fixing.

If your negatives are clear or too thin, it's not your fixer that is the problem; that lies in exposure and development.