You do not have that much of reciprocity failure for times around 2 seconds. So even with Foma 100 that only ends in the 5 seconds range.
Using times which already have reciprocity correction and then go to a different film speed does not help. You have to change the measured time and then do reciprocity correction. Reciprocity correction is not simply a factor which you multiply your measured time with.
Your measured time is 1/2@f8 which translates into around 7 minutes for f207. Add reciprocity will leave you at around 40 minutes.
Now take a film which is two stops slower and assume it has the same reciprocity behavior as HP5:
Now your measured time is 2 seconds @ f8 which translates into 28 minutes @ f207 and with reciprocity you end at around 5 hours and 30 minutes. Which is different from the 2 hours you would get if you calculate for the slower speed and start from the already corrected time of 40 minutes.
And then there is the fact, that different films have different reciprocity behavior.
Just to have a fun start I would recommend Fuji Neopan Acros which has nearly no reciprocity failure up to 120 seconds. And up to 1 hour it is less than a stop.