Who left the stain?

The Gemara in nida on daf 60b explains the Mishna that if two women were lying on a bed and they get up and there is a kesem on the bed (bigger than a gris) then one can't assume that it came from the other and they are both temeios. However if one of them is nursing and one isn't then the nursing woman can assume that it came from the non-nursing woman and only the latter is temeia. Same thing if one is pregnant and one isn't or if one is old (post-menopausal) and one isn't or one is a young girl and one isn't. The gemara concludes that if they're both pregnant or both nursing or both old or both young then there's no reason to assume it came from one over the other so they're both temeios.

The gemara does not talk about a case where one was pregnant and one was nursing or any of the two women in different categories. What would the halacha be in that case? For example, if one woman was 80 years old and the other is nursing can the 80 year old assume it came from the nursing woman? It's true that it was unlikely for either of them to see dam but now that there is dam isn't it much more likely that it came from the nursing woman than from the 80 year old? How much more likely does it need to be to blame it on one woman over the other?

Tosafos on the bottom of 60b doesn't address my specific question but he does address two other similar questions. If one woman is close to shaas vesta and the other woman is not can we assume that it came from the woman who was close to shaas vesta? What about if one woman didn't have a regular cycle and the other woman did (and it wasn't that time of month) can we assume it came from the woman without the regular cycle? Tosafos says that in both those cases you can't blame it on one over the other.

I'm not sure if I'd extend Tosafos to our case. I guess it really depends how much more likely it is that it came from one woman over the other. In the case of the gemara it's about 90% likely so we assume it came from that one. Tosafos' case is like 65% (I'm just making up the numbers to illustrate the point) likely so we don't assume in that case. What about in my case which is probably somewhere in between? Or maybe it doesn't matter how likely it is but only in these cases where one has a reason to assume that it didn't come from her but the other doesn't have that same reason will we say it. If however they both have a reason to assume it didn't come from them even if one has a much better reason then we cannot assume it came from the other. I looked at some of the Rishonim and didn't see anyone address this.