It's quite difficult to know exactly. If you are a "Stand Up Guy", which is a cheesy joke I used to make that refers to people that can plugg and stand very soon thereafter. Plugging while standing is guaranteed to drip some just from the syringe removal.
Laying on your side or however is most comfortable then standing up is probably your best bet if there's no leakage.
Laying on your side the whole time is comfortable but most of the solution will be absorbed by the hepatic portal. I think the best is to try and get as much as you can into systemic circulation knowing that's it's probably only going to be 30-60 % if you are excellent with your technique.
The area that actually goes straight into systemic circulation is only about 5cm from the bum hole. so inserting the solution then attempting to get it to come back down to the lowest point possible is your best bet. At least some is making it into systemic circulation so it's a win even if you only get a little.
Most importantly one doesn't want to 'leak' any back out. I won't get too far into it (get it ?, lol I kill me) but if one deals with reg constipation or certain lifestyle choices, it might be more difficult to keep liquid low in the rectum while standing, If one is young among other factors/ lifestyle choices, then they might be a "Stand Up Guy or Gal". Again, even if it's only a little from laying on one's side then sitting up for a bit, it's better than nothing.
Here's an idea: Only the blue area at the bottom of your bottom goes straight to systemic circulation, but again, even a little is worth it and the absorption into the vessels leading to the hepatic portae is at least rapid. See what you think..
