I went through this same issue. What I ended up doing was this....

Originally, when getting tattooed and measure for the jig, I had trapped gas and a full bladder. Tech asked me if I could pass gas without peeing, ummm nope. So he called the doc and the doc essentially put a tube up my anus which released a surprising amount of gas. What little dignity was left, right out the window. 🙂 So if you're having trouble, they can have a plan "B". This was the only time I needed that assist...

My radiation treatment was at 9:00 the center was really good about keeping to schedule, so that was a big help

I'd get up about 7 ish, have a cup of java, then morning constitutional sometime in that first hour. Then I would drink about 24 ounces of water over the next half hour. I didn't eat anything during that time.

If I had to release flatulence during this time period, I'd go to the bathroom and stand over the toilet. Only let out the urine I absolutely had to when it happened. It didn't happen much because I've moved my bowels already and have not eaten anything at all.

Using this formula, I was always about 80% (often closer to 100% ) bladder capacity and had empty bowels. In the 39 treatments I only had to hang around a few minutes and drink water once. After zapping I would eat breakfast in my vehicle then drive directly to the gym and have a work out.

I realize everyone's a little different and I was VERY lucky where time of day was concerned.

Regarding the flatulence, unfortunately, this minor annoyance happens to this day. I describe it like this..

Prior to surgery, if I felt gas I thought I could get rid of, and was alone somewhere, I'd push to get it out, I think most folks do.
After surgery (almost 2.5 years) , if I do the above, I can still have a minuscule leakage, it's about a 50/50 at this point. What I have learned is to let it out, as opposed to push it out. This has helped tremendously.

It gets better over time....or at least it has for me.
Best of luck to you!