I would stop adding the Borax and give the pool a couple of days for everything to "settle out" and then retest....7.8 is a little high but not enough to cause you any problems.

Stabilizer does have to be added separately...the difference in readings is probably due to difference in testing (different light, different background, letting the water/reagent sit a different length of time, etc.). It is a very subjective test and only really gives you a ballpark, anyway. If you haven't added any more, then just go with 50-60, and that will give you an idea of where your chlorine needs to be. At 60, keep your Chlorine at minimum of 3 and max of 6. You do need to keep adding bleach to keep it at that level--don't let it get below 3.

Janet