Leevi is exactly what we need right now, an RFA goalies who can be bridged through Ullmark's contract, and then get his big payday in 4 years.
Unless Ullmark doesn't come back and Leevi plays 30+ games at .915+, they can likely get him at 1.5M-2.5M on a 2-4 year term. Going 4 years off of a very small sample size would be risky, but if it works out it means they have a tandem locked in at around 11M and don't have to worry about paying 4M-5M for a legit #2 goalie if Ullmark can only go 40-50 games per season.
Ullmark is clearly a tandem goalie. Most teams use tandems now, but not every goalie wants to only play 40 games. I get the impression looking at Ullmark's career that he prefers that situation.
Kotchetkov in Carolina might be a comparable to look at. He was drafted a year earlier and signed 4 years at 2M with a limited NHL resume. If they go 4 years, I'd expect it to come in a bit higher, and I don't know that the Senators would want to risk that kind of term off if what might end up being a 20 game sample size.