A few things to note.
Billing at 25 min is difficult because it is not easily divisible by 60.
I bill in 30 min increments for remote work and 1 hour increments for on-site work. I have a 15 min "keep in touch" period which is unbillable if used to alert me to problems and seek advice on whether tech support is needed or not. In short if you are my customer you can call me up with a problem and I will spend up to 15 min figuring out if you need to buy services from me and if so what. Then we can talk. On-site I offer last 15 min free but the increment is hourly (and for longer trips there is a minimum billable). The idea is that if I am there for up to 1:15 I bill for an hour. If I am there for 2:05, I bill for 2 hours. If I am there for 2:30 I bill for three hours. This is in part due to travel time, and customers are aware of this up front. I also offer to spend a little more time answering questions so there isn't a lot of time they pay for that they aren't using (it's a nice way to upsell too where appropriate).
you want your billing to be easy for customers to understand. If you are dividing your time into 25 min increments, you should still consider billing on the half hour and tracking those separately from how you bill.
- 1 25 min time is 1/2 hr
- 2 is 1 hour
- 3 is billed at 1.5 hrs
- 4 is billed at 1.5 hrs
- 5 is 2 hrs
- 6 is exactly 2.5 hrs.
So you can cycle groups of 6 this way. Moreover it is symmetric. After 1, you owe your customer 5 min, after 2, 10 min, after 3 15 min. After 4, you pay back 15 min, and return on the other side back to 0. So if you do a lot of short tasks you make a little extra, and if you do a lot of longer tasks, you pay it back. This can be a nice incentive to use more of your services instead of a little bit here and a little bit there.