I have a 21' boat and plan on getting a BIMINI top for it (sunbrella). I am
concerned on the measurements,

First i plan on mounting the main frame on the outside of the windshield,
good or bad?
Second, how exact do the measurements have to be, since i have to order it
online i'm worried if something is exact i'll have a problem
third, i am hesitant about installing it myself, drilling into boat, how
difficult and should i have a marina try to do the install?

Thanks! !