I normally build a simple fine scale simulation model and match, so dont have oodles of experience with this. I'd suggest that simple extrapolation could be achieved using Horner type approach, the key assumption is what pumping time to use.

Horner extrapolation is normally used for PTA, as such a steady (or pseudo steady) state system is a different beastie. If your well has been producing for (say) 2 months, then using a 2 month pumping time with this approach would not be appropriate, as most of this period was not transient (Assuming you are not talking about really low perm systems).

I would hazard a guess that if you calculate (or even better, through historical production behaviour infer) what the time required to get out of transient flow (ie how long the initial 'flush' production phase is before your production follows a normal depletion trend), and use this as the pumping time tp

Then plot Pressure vs log10(ts / (ts + tp)), where ts = shutin time

And extrapolate to ts / (ts + tp) = 1