1st i dont think there are typical ratios for bulking and cutting. each person has his own ratio and its typically only born out thru a decent amount of time lifting/eating seriously (perhaps 6months to a yr depending on how serious). by that i mean a real effort at a legit lifting program and a real effort to get in sufficient cals as well as protein etc.
after that i think its quickly apparent where your p-ratio falls. 60/40 is prob. on the better side of genetically avg imo. not outstanding or typical pro athlete level but better then avg.
i think the main thing to keep in mind is how much you can actually affect your ratio.......even with all the variables you mention being dialed in your looking at a 10-20% improvement. so if your ratio happens to be 50/50 then doing all you can "just right" will at best move you a little above that 50/50 maybe even to 60/40. of course "just right" can and does vary person to person so it takes time to figure out how your body responds to all the variables involved in bulking or cutting. this is why folks who cut or bulk (for real) for the 1st time are rarely happy with the results. usually the 2nd and 3rd attempts are more successful as people learn to keep what works and discard what didnt.
as far as your last question. i think 90/10 is very very optimistic but still possible. again, i think a person would need a pretty good understanding of how their body responds to whatever diet/workout plan they where going to use. picking a good diet strategy that is effective (say ud2.0 or IF etc.) will most likely yield good results but i doubt 90/10 on the 1st try. perhaps on the 2nd or 3rd once youve figured out the nuances, how you resond, worked it into your lifestyle, sched, etc.
here is a link to one of the chapters in lyles ud2.0 where ive gotten most of my info about p-ratio.
http://www.wannabebig.com/article.php?articleid=125
finally, as far as what is an avg p-ratio.......i couldnt say. i could venture a guess but i dont think it would really matter. i have a pretty good idea of my personal ratio and thats the only one im sure of. i cant really change it so i just have to adjust to it and be realistic about my goals or at least the time it may take to reach them.
i hope that is of some help
good luck