737 takeoff performance calculations
 

I have troubles understanding t/o performance calculations when it comes to derates and assumed temperatures. I use PMDG 737 (sooner or later I might need to solve that IRL) and their manuals which are pretty much copy-paste Boeing's FCOMs.

Example:
TODA = 2000m
Wind: calm
Slope =0%
Airport elevation = 0ft
Runway: DRY
Temp = 12C
-----------------
Flaps: 5
Let Assumed temp be 42C
and 22K derate

My way of doing the calculations:
In FCOM vol 1 I go to page PD.30.1
because of the wind strength and 0% slope TODA = corrected field length
on PD.30.2 I can see Field Limit Weight of 64.4t (no derates and assumed temp included into calculations? correct me if I'm wrong)
on PI.30.38 I get N1 = 91,7%
From here I can calculate V1/VR/V2 based on first two tables on PI.30.28.
How can I get Field Limit Weight for that derate and assumed temp setting?