Phil,
if I am teaching you to suck eggs here, then sorry just ignore my ramblings....

to measure your upthrust accurately on the c50,

stick a prop on the engine, (bigger the better) and have it vertical (inline with the fin and rudder)
measure from prop tips to the back of the fuse at the top of the flat sidesjust in front of the rudder (ie a point on a horizontal line that should pass through the center of the prop)
divide the difference of the two measurements by the length of the prop
take the inverse tangent of the above

Can also be used to measure side thrust, just put the prop horizontal and measure to a point on the fin.