Saddle height? Inseam minus 10cm?

Saddle height = inseam x 0.885

Here is saddle height = BB centre - top of saddle

Of course it can be different for each person and for different conditions, but it's a rough guideline.

Been feeling like perhaps my saddle is a bit low... so had a quick trawl through past posts and found this.

My slightly dumb question is... is the height calculated on the perpendicular? IE measure from centre of BB straight up at an angle of 0 degrees, to the top of the saddle? Or, should I measure along the tube, which looks like it runs at 5-10 degrees? There's a 4cm difference by my calculation.