kind of...

the language actully says 50% add pay of the scheduled leg time. So if one DH's in a middle seat on a 2 hour flight, 1 hour of add pay is earned. Add pay is a seperate pay bucket (includes vacation and other incentive pay) seperate from the line or trip value buckets which can fluctuate based on trip trades or cancellations etc. Add pay is static.

And you actually have to be assigned and sit in that seat. If for some reason you aren't already assigned a First class DH per the contract, and get a middle seat but then get upgraded to first class at the gate, you don't get the middle seat add pay.