So I need to set a straight piece of something rail to rail to neutralize the concave or Vee , and get my cant based on a straigh line?

So my board that has 3/16 tail concave and 97 degrees of cant(measuring from concave) , actually has 95,5 degrees of cant (measuring from straight line)?

A: yes, but you could put a proctractor to the other side as well

>So I need to set a straight piece of something rail to rail to
neutralize the concave or Vee , and get my cant based on a straigh line?

A: Yes , or flat bottom.

You using glass ons? or what?

>So my board that has 3/16 tail concave and 97 degrees of cant(measuring
from concave) , actually has 95,5 degrees of cant (measuring from
straight line)?

A: it doesnt have 97 degrees or 95.5 degrees of cant (your from Europe?), it has either 7 degrees of cant from the 97 measurement or 5.5 degrees of cant from the 95.5 measurement.

When a fin is set at 90 degrees it is said to have 0 degrees of cant.