How much would you dock from (or add to) a 3.2 Carrera for...
 

a cracked dash, Recaro seats, and RS door panels?

start with what it would cost to make those known issues right....dont forget to get a PPI done.

A cracked dash tells me that the other interior bits are probably needing replaced as well. Used full interior might cost $2500. Having it professionally restored (interior) with correct everything..$10k.

EVERYTHING on these older Porsches is more expensive than it was 24 months ago.

That can be a useful tool in a negotiation, but carried to its logical extreme, trying to itemize things that need "fixing" is probably going to price you out of any 25 year old car. After all, pretty much every 74-89 car is worth less than the sum of its parts.

The same way that maintenance & upgrades don't return value on a dollar for dollar basis, neither should needs deduct on a pure "replacement cost" basis. However, I think an itemized deduction is more appropriate for things like aftermarket seats and wheels than for a cracked dash, worn (but serviceable seats), chips in the paint etc.

If the Recaros are nice and suit the interior, maybe knock off $500-1000, recognizing that you can sell the seats to offset the cost of buying a new set. Maybe $300-500 for the door cards. Lean toward the low end if the interior is black, as the seats & cards are easier to source. If it is burgundy or a more rare interior, it may be difficult or expensive.

Apart from the bolt on pieces, I think you have to take the age/wear/patina/condition as a whole, and assess how it might affect value.

If the dash is cracked, but the interior and paint are otherwise exceptional, you have to balance the dash against the better than average condition of the rest of the car.