Depends on the trailer. You're right to be thinking about break-over angle, but you also need to consider your front lip hitting the ramp, or dragging on the ground behind if you back it in.
What is the load deck height & how long are your ramps? Most will need some kind of accomodation if the trailer doesn't lower or tilt to aid loading.
I put mine on a U-Haul car hauler, BACKING it on. But the other key is extending the trailer ramps to reduce the break-over and and approach angles. The U-Haul car hauler ramps are 6'6" long and I extend them to ~10' 6" with these extra "maintenance" ramps. $100 well spent. The longer the ramp, the easier it is on your clutch as well.
www.canadiantire.ca
(U-Haul recommends building steps out of stacked 2x8's but the steps are hard to drive up and hard on your clutch - this works much better.)
I converted the "maintenance" ramps to ramp extensions by sawing off the lip at the far end (that prevents you from driving off.) Then I position it so that the car hauler ramp rests on the last 2" of the plastic extending ramps. It is a bit of a blip going from the ramp extensions onto the hauler ramps, so I'm thinking of making small blocks to line the hauler ramps up exactly with the top of the ramp extensions.
To answer your question in degrees, my 10' 6" ramps with the 19" U-Haul deck height would average 8.6 degrees, but the maintenance ramps are 10 degrees so the U-Haul ramp portion is ~8 degrees? (My NSX also has ~4" clearance, measured at the jack points.) The stock 6' 6" ramps work out to 14 degrees. I tried that once but stopped because I could see it was going to scrape.
The extensions pretty light & easy to throw in the back of the pickup, but they're rated for 16,000 lbs! I can't find a pic now, but I'll take a pic next time I'm loading it. But that will be a couple weeks and I infer you're doing it sooner.
You can also back up to a ditch, a hill, or a large curb to create a pretty much flat loading situation.