A skill chain becomes Ultimate when the total, before the multiplier, hits 20,000. For instance 20,000 x3.2 is an Ultimate. The multiplier doesn't matter.
If you go for this in an area with jumps you can combine these:
Grab a Skills boost HE car if you have one. Either way, head out west to the coast. Between all the small bumps giving you Air skills and the foliage giving you Wreckage skills, plus all the combinations, this should go quickly. Just get to 20,000 then stop and bank it and repeat.