I understand your objective and agree with your plan.
Glad you mentioned the case in question.
I have not reloaded in decades, but got a jar of trailboss with intentions of trying some large rifle cases.
back to your plan, the possible "problem" I see getting VERY accurate measurement will be putting powder through the small hole and then about its settling to fill the case.
Using water to fill the case through enlarged primer hole with a bullet seated would give VERY exact measurement of 100% case volume for YOUR cases and loading.
then you can get your exact 60% for starting trailboss loading.
For my intented trailboss loading of large rifle (first being 30-06), I was able to find online case volume for "loaded", and done all the math to get back to the Lee scoops to get my starting and max loadings. the scoops supplied with my classic lee loaders look like will provide just the right amount of trailboss too.
when I actually get around to putting rounds together and shotting a couple over the sky screens I was going to post my notes
IF your 10mm bullets are flat based, how about figuring out the depth into a case and then cut a case to make a scoop?