I plan to take a taxi like Bolt from Bangkok to Bangsaen and later on the same day back again. 

I understand that the prices vary depending on a couple of factors.

Are there any (official) predictions about the prices through the day and maybe workday or weekend.

I.e. I checked this morning about 8:20 and the Bolt standard price for above trip was 2400B. Now, 10:10, it shows 1230B.

In November I paid for that trip on a Sunday 1017B and then back 794B.

 

I am flexible with my time. I don't need to concentrate on the last 100B difference, but I prefer to pay 1200 instead of 2400.

Is there an easy way to predict when to go for a reasonable price?

The whole idea of price discrimination (nowadays often a different price for every customer) is to be unpredictable and thus squeezing the last 1200 baht out of the customer. 

There probably is a correlation between the estimated travel time and the price.

I came back fron Rangsit Northern Bangkok (not too far from Don Maung) couple weeks ago to Jomtien

 

Base price was 1215 and there was no attempted loading on price.....paid for tolls and added tip which in total was 1450

 

It was around 11am in the morning