I don't know for sure (I've only got the .40), but I'd bet that the caliber of your P99 makes a difference in which front sight you use.
I'll let you know what I find out when I get my Meprolights in the next week or so. I'm currently using the #5, and my shots are hitting a bit high, so I think the #4 is going to fit my bill perfectly as well. I'm going to the range tomorrow, so I'll know for sure.
You're entirely right, and I realized the same while I was at the range today. I'll be posting a range report in the next day or two (when I get some pictures taken). I ended up using the #6 sight to get my POI right on my POA.