I never got any problems like it on FF so I think it is Chrome specific. Happens most to me on Batracer and The Student Room, only fairly rarely on here. Refresh always works.
Probably better to do it by percentages. Being 2 second off at FE1 isn't that great but being 2 seconds off at KY3 isn't too bad. Doing it by percentages, each % is the same as 1 second on a 1.40 lap. Anything less than 102% of the fastest isn't too shabby