Straightforward once you see how a trimsheet is designed.
How about you link to a graphic of the trimsheet so we know what you are starting with .. and we can work you through the exercise.
This sort of question arises from time to time .. so there are those about who might like to play with the same thing in the future.
So far as accuracy goes, if the trimsheet has been designed sensibly, and we have the TCDS/AFM/POH etc. it will be fine. Indeed, for a well designed trimsheet, the accuracy is of practical equivalence to a manual calculation.