What is the effect of low temp. on hop utilization?

During my last brew session, I managed to run out of propane with no spare cylinder and about 20 mins left in a 90 min boil. I just continued as planned figuring I must be getting some kind of hop utilization even if it wasn't boiling as the temperature at the end was still about 190 degrees. It'll be a couple of weeks before I see if I have anything drinkable or not, but in the mean time, is there some way to guesstimate my IBUs?

you're good. you did an extended boil so as long as the bittering hops went in during the beginning of the boil, i'd say you're at your target, or close to it. if you're saying that you added an 1oz at flameout hoping to have boiled it an extra 20 mins, you're not going to get the IBUs from that last addition. what you will get, though, is aroma.

Use beersmith or one of the online calculators to model what happened and see what the difference is in calculated IBUs. If you aded hops when or after the flame went out, you will get some IBUs and, depending upon the hop, lots of flavor.

I had something similiar happen to me last brew -ran out of propane 15 minutes into the boil. Took me 20 minutes to get another tank and resume boiling. I call it doing a "boil rest" haha. Beer seems fine.

IIRC, there is some hop utilization >150F, the rate just slows down alot the colder it gets.