No you're half right. If you have 915 HP at level 6 with no HP items you gain 250 HP from ult, putting you at 1165 max HP.
If you had 200 HP (below the 25% threshold of 228) and ulted, you get 450 out of 1165, which is above the 25%. HOWEVER, I believe once the game codes you as "able to pull" it might be calculating from his non-ult HP.
Good way to test this? Have a custom with Urgot vs Nasus Renekton Soraka Sona Nidalee. Have the Urgot lower Nidalee to 20% HP, mark her, VERIFY she can be pulled via the icon, and have Sona Soraka and Nidalee heal her, see if she can still be pulled.
Next step is to repeat but with Nasus without ult, then repeat with ult. This is to see if it happens with his ult too. THEN repeat with Renekton.