Guided by your explanation then; if I want to burn an area for an additional 1/2 stop above the base, 10 sec:
If I understand correctly. If, for example, your base exposure is 10 sec and you press -1 stop, the timer will count up to 5 secs. By pressing the calc button it will show this. If you then pressed +1/4 with 'Calc' button on it then shows 11.9 sec which is the base, 10 secs, + the 1/4 stop. Your base time is always set in the timer via the keys under the display and stored by the 'Ent' button. Personally I have found this timer to be one of the best f / stop timers. I have never tried the RH stop clock but do have the Analyser. I find that good but it has to many secondary functions which confuses my simple mind.