I've narrowed it down to needing two underquilts for my area (temps ranging from hot humid over 100 in the summer to wet rainy only supposed to be 20 but was 17 last winter). I flat out can not use just one quilt to cover that type of range in temps. I too sleep very cold and must subtract 15 degrees from the rating of any quilt. This means that I needed a 0 degree quilt for that night that hit 17 degrees. This 0 degree quilt works well for me in temps up to about 50. I do have to vent it as the temps go up higher. My lighter under quilt is an IX 3 layer quilt. Some people have had good results with this quilt at lower temps but not me. Because of my cold sleeping, my IX is a great warm weather quilt. Plus because it's so humid here and IX is waterproof, it works very well. And it was cheap!! Meaning I could spend more money on that winter 0 degree quilt!!!
I decided that for me, I'd rather be to warm and need to vent, rather than to cold and shiver all night. And your question about can you use a 0 degree quilt in higher temps... this is something that you will need to test for yourself to see how high the temp can go before you are to hot. Even I don't use my 0 degree at 60 degrees, it's way to hot, even vented. My IX works well at that temp though.