B737NG PILOT:
I'll add my tuppn'yworth for your consideration. First: DO NOT rotate into the FD. Rotate to an attitude; i.e. look through the FD. The target is 15degrees. The FD commands a minimum of V2+15 to a max V2+25. This is on climb-out after the gear is up. Hence the initial target attitude. You will find that if you select a thrust which equals your weight, i.e. you use minimum thrust the FD will stabilise at 15 degrees eventually. If you over rotate the IAS will be <V2+15 and the FD will lower the attitude to accelerate to V2+15 then climb-out at that speed. If you under rotate, or rotate late, the IAS may be >V2+25 and the FD will eventually guide an increase in attitude to reduce IAS back to V2+25. It will then most likely stabilise at 15 degrees again. If you rotate properly you will find that you are at V2+20 and the FD will maintain this with 15 degrees NU.
If you use more than minimum thrust the initial rotation is still 15 degrees, but the excess thrust will allow a climb-out at V2 +20 with higher than 15 degrees. The FD will guide this attitude. It looks at many performance parameters to give correct guidance.
If you rotate such that the speed at liftoff is >V2+25 beware and DO NOT rotate to FD's as this will cause a rapid drop in IAS and a porpoise pitch climb-out. Above 20 degrees the drop in IAS is VERY rapid; and why would you want to go above 20degrees? Either you have screwed up the rotation and be at too high IAS, or you are way over powered. Either way your initial climb-out trajectory will be more than enough. If you are at so high an attitude the IAS will reduce quickly and then the FD will command a strong reduction in attitude to maintain V2+25. This can be quite uncomfortable.
A single continuous rotation at the correct rate to 15degrees, started at Vr should result in V2+20, and after the gear is up you'll find the FD maintains this with 15 degrees is weight matches thrust. Be aware if you are over powered and then the resulting extra 1 or 2 degrees is not confusing. BUT always use the initial 15 degrees.
Note" this is for F5 takeoffs. F15 may be 1-2 degrees lower.