Z height wrong after G29?

I run G28 and the Z position is fine ~.1mm from the glass in the center of the bed so I know my probe offset is good.
After I run G29 after and it plots its points it ends up being about .2-.4mm above the glass when I ask it to go to Z0 position in the center of the bed.
Im guessing my glass is shaped like a bowl for this to happen? Should I just change the offset to accommodate?
This is a fairly thick piece of borosilicate glass on a MK2 PCB heater. Temps get to 110c and hold with no issues on the bed.

Ive been starting prints and having to babystep them down on the skirt to get them to print right. Id rather hit print and have it just work.

Nothing out of the ordinary from what I can read.

First is not the glass but rather the PCB board that bows under heat, also your rods might sag at the middle due to weight. But when you set your layer height 0.1, 0.2 etc it will be added to the existing 0.1 distance you have with g28. So a 0.3 layer height will translate to a 0.4 once you account for your initial offset.

Best way to correct this will be setting your layer height and without filament on the printer hit print and then stop it as soon as it does the first print lines, using a filler gauge measure the gap and deduct the distance from current sensor offset.

Edited 1 time(s). Last edit at 11/12/2016 07:47PM by ggherbaz.

Sounds good, I will give it a try.
I was measuring probe offset after G28 not G29.
Ill set it after and see

After setting up G28 then G29 I measured and adjusted the offset and now its spot on.
Dont know why I didnt think to measure after the G29.
Oh well you learn a little every day.

Glad to know, just one more note: the adjustment might change depending on temperarure settings (ABS vs PLA) the PCB board will bow more at higher temperatures so be prepare to change offset values whe switching materials, but you can do it via slicer instead of firmware.

I only use ABS so thats not an issue