Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison
hard to say, u talking left chest prints or full backs.
Rutland, Wlflex etc have calculators. Large margin of error do to almost infinite printing variables. For the 10K to 100K orders I have the ink dept print each ink/screen separately and weight the shirt wet. Or pellons. Whatever. Just tare first of course. This is the most accurate method if you really want to put cost on large projects
If it's after the fact - or a project to get the best approximations possible - you could weigh the ink before and after, don't forget to estimate the loss due to screen / spatula / squeegee / floodbar cleanup also.
because the cost of the ink is so low per print it's not worth worrying about.