Nappa, when regaining his cool, proved to be Goku's 8k rival, so ~7,500 seems good to him. Dodoria was easily defeated by 24k Vegeta, putting him considerably below this level. In spite of it, he still should be noticeably above 18k Cui/Vegeta, as he was a rank above them in Freeza's soldiers. 20,000 seems a good apporach. Said that, Kaioken x3 Nappa would end up noticeably above Dodoria (22,500 vs 20,000) and the battle's outcome would depend of Nappa's strain with the Kaioken. If he can sustain it as good as Goku in the fight against Vegeta, I'd put my money on him, considering he is also a durable beast.
Kuririn (fight against freeza) vs Captain Ginyu