The prorating is based off their remaining value. They lose 2 days worth of value because they used 2 days already. Possibly 3 given all the calculations are in UTC. The value per day is roughly 40 cents for your 149 plan. That means they lose roughly 80 cents towards the 199 plan. I'm guessing they actually hit 3 days UTC as 3 days would put the plan roughly at 51.2. This all seams to be calculating correctly.

Per day Value: 149 / 365 = 0.40821
Used Value: 0.40821 x 2 = 0.81642
Credit: 149 - 0.81642 = 148.18358
Prorated Upgrade: 199 - 148.18358 = 50.81642

It's very close to your 51.17, which is why I think it's actually closer to 3 days worth of time used as it calculates in UTC. This would be the below.

Used Value: 0.40821 x 3 = 1.22463
Credit: 149 - 1.22463 = 147.77537
Prorated Upgrade: 199 - 147.77537 = 51.22463

This is just using flat days. CBSubs will calculate down to the very second since it's a timestamp calculation with a rounding to the 5th decimal. So yes it appears to be calculating perfectly fine.

It sounds like you want to prorate the time they used towards the upgrade, which CBSubs does not do as I've shown above. The parameter name and description both clearly state they prorate the remaining value.

Don't understand what you're asking. The plan is 1 year at 149. So the remaining value is calculated based off what they paid vs the time left on their subscription that they did not use. You can see this above in my calculations. They do not get the value of the time they already used prorated towards their upgrade subscription. That's why it's not a flat 50 upgrade, because they used some of their time already.