Finding out Film Dev Times

Is there a way to mathematically calculate a base time for a film+iso+dev+dillution combo if you have a known time in different dillution or iso? For example, I have the time for Ilford FP4+ at 64iso with Rodinal 1-100 which I have succesfully developed in 19 minutes at 21C. How can I find the times for other dillutions such as 1-50 or 1-25? Or another example would be the Agfa APX 100 which I want to rate at 50iso yet the only times I have are for 100iso.
The reason I ask is that I lack a densitometer to precicely test developed negatives which because of slow iso and long exposure I wouldn't be able to judge by eye correctly.

Easiest way I know without a densitometer is to use a step wedge and process for a print on the paper of your choice. Once you have your film speed, it is just a matter of timing development to match zone VIII for highlight values. While not as scientific, it will still give you results tailored to your film, developer and paper combination.