How do you determine proper frame size these days given the sloping top tubes of most bikes? Are the old formulas of .65 x inseam for c-c and .67 x inseam for c-t in any way relevant?

My inseam is about 86 cm and I've been sized on anything from 53cm to 56 cm. I know that there's a range of frame sizes most people fit, but is there a formula used now days for the sloping TT? Thanks.