I'm convinced this had much to do with my starting pH which was later measured to be 7.0.
The water pH has very little to do with mash pH. It is the alkalinity that determines where the mash pH will go.
Looks like on brew day I need to calibrate and measure tap water pH instead of assuming the water report pH is up to date.
Your source water may well be variable and the fact that the pH is different from day to day can be taken as an indicator that this is happening but if you want to compensate for change then you must measure the alkalinity at each brew day. This is easy enough to do with inexpensive kits which can be obtained from aquarists or suppliers such as Hach.
Another challenge I seemed to face was determining what value to record when using the Ph meter. The display just didn't seems to settle. When I say this it wasn't jumpy rather shifting 0.05 within 10 minutes. Is this unusual with a meter that has a resolution to 0.01?
No, it' normal. See the pH Meter Calibration sticky.
Some posters here have had terrible luck with these and for others they have been fine. See the pH Calibration sticky for tips on how to check the stability of your meter.By thy the way I'm using the MW102 from Milwaukee.
There is a bit of a learning curve with pH meters as there is with almost everything else. Practice with your meter on various food items around the house: vinegar, milk, lemon juice, baking soda solution, tap water, coffee (cooled), soda, orange juice...I need to prepare myself for next time.
I'm not sure why the CaC03 read -98 and the RA at -160. Can this be possible?
Not at a pH of 5.48. There must be something wrong with the program or calculator you used. Alkalinity is the amount of acid required to reduce the pH of 1 liter of water to a reference pH near 4.5. If you were given a sample with a pH less than the reference pH you would have to remove acid (add base) to bring the pH to the reference pH and the alkalinity of that sample would be negative.
RA = alkalinity - [calcium_hardness + magnesium_hardness/2]/3.5. Thus if the alkalinity is negative or less than [calcium_hardness + magnesium_hardness/2]/3.5 the RA can be negative. This does happen for the second reason but not for the first.
Doubtless the reason you are seeing the negative numbers is because of the addition of sauermalz but the program/spreadsheet/calculator you are using apparently isn't handling the calculation properly.