You actually asked two or three questions here as 'planet ice caps' includes a few terms:
Sea Ice - When it comes to northern hemisphere, sea ice is a major component of the discussion.
Glaciers / Ice Sheets - Mostly a discussion involving Greenland and Antarctic, but involves northern Canada and Russia as well.
Perma-Frost. Frozen bogs/marsh/tundra
http://nsidc.org/data National snow and ice data center is a good source of information on this. In particular, http://nsidc.org/data/tools/analysis-and-imaging
NOAA is another good source to use for this discussion.
Google searching NOAA ice cap coverage and similar terms will find these results.
Not sure if you will find equations however...these are measurements and observations, not hard formulas by any means.
Edit in from comments:
Ah, I missed the mark with my answer then, I figured you were talking earth sea ice over time, not a fictional planet.
I don't think you'll find any formulas along these lines...remember something as complex as the el nino / la nina cycle that we understand so little about (it's normally observed by temps in the southern pacific) has a big impact on the sea ice in the northern hemisphere. Solar cycles also play a role here, and we hardly understand those. This doesn't get into England, which by latitude should be iced over much of the year, but Thermohaline circulation keeps it relatively warm. How can any formula manage to take into account something as complex as that?
With a hard science tag on the question, there really is no answer that can be given as we understand so little on the subject...if you do find a formula on ice coverage based on temp and water coverage, I'm pretty sure randomly rolling dice and guessing is about as accurate.
My best advice I can give here is to simply say it can be anything you want as a planets climate is ever changing and never a set thing. Remember, you are taking a snapshot of a worlds climate at a particular point in time...it could have been just entering a cooling cycle and hit those average temperatures, but still be relatively ice free as it's just entering the cooling cycle. It could be just ending the cooling cycle and the ice caps could nearly reach the equators. Where it is along this cycle is really your choice as it's your snapshot of the world.
With only 20% water coverage, your planets temperatures are going to be relatively susceptible to rapid changes over a few years...this is really whatever you want it to be.