Effect of slope (left to right) on dip-stick oil level measurements?

In my garage where the prius is parked it is fairly close to level from front to back but it slopes noticeably from left to right, with the right hand side (the side the ICE is on) being higher than the left hand side.

Does anyone know roughly how much that will throw off any oil level readings I take from the dip-stick while its parked there. The reason I ask is because I often don't get the opportunity to test it properly on flat level ground like I really should. I'm wondering if measuring it with the right side higher than the left would tend to make it under-read or over-read or neither. Any ideas?

 

Good idea. I can also drive it somewhere level and check it there and I will do that when I get a chance. Anyway I was just wondering if someone else has already tested this and perhaps knew how sensistive (or not) the oil level is to L-R slope.