Measuring the legs of the triangle instead of the hypotenuse in Fusion 360 sketch
For this, use a construction line:


draw the approximate line
press x to go into the construction-line mode
pres l for the line tool
Draw a line from the end to the base
constrict it orthogonal to the base, turning it into the height
press d to go into defining distance mode
click on the dotted line
type in the height
press x again to get back to normal lines.
complete the triangle