Sounds as though your on the correct track there, I always use a meter when building a wing ,after I get the frame work done I'll block it up and measure the wing with a meter, if everything is ok I then proceed to the sheeting ,if its not I'll shim it so its past the point of being straight then sheet if its not to bad. if its to bad I will soak it and pin it past the point of flat and let dry recheck things and if eveythings a go then go to the sheeting.
1/2 a degree warp is nothing and you can easily take that much out with the covering and heat gun,I have pulled as much as a 1-1/2 degrees out just with the covering depending on the type of wing it is, But its tough to do and takes some time and as a rule 2 people. Make sure to wear some heavy gloves when doing this otherwise a burn or two can happen and fast.

Now if your building a fully sheeted wing your not going to be able to pull much out with the covering maybe 1/2 degree and thats a big maybe, those type of wings I want as straight as possible before I sheet.
Good luck Richard.