heated bed clamshell chip

for the second time I have had a huge clamshell chip peel off from my heated bed when printing a large flat part, after the first time (when the glass also cracked), I replaced the bed, this time I went for the cheaper option and purchased some 3mm borosilicate glass sheet 200 x 215mm this allows me to place the replacement glass over the chipped bed (almost, it really needs 215 x 215mm to completely cover the bed).
it has cut down one edge by about 15mm (I placed the sheet under the hold down plates so the edge nearest the origin aligned- so the further edge comes 15mm short).
I did a test print last night which has worked well, so its just a matter of testing now to see how well it stands up.
I bought two sheets of the glass for £19.99 so used the second sheet on my spare (old cracked) heated bed. I could have bought a 4mm thick sheet 215 x 215 for £26, but thought I would test with the cheaper option,  besides, how often do we use the whole bed size, and if these sheets of glass also clamshell they are cheaper to replace.
The hold down screws accepted this additional 3mm, I think if I went to 4mm I would have to use longer hold down screws in the corner brackets.
A manual re-levelling was done using the paper sheet thickness method.