Brake pads - how worn is worn-out?

[Smartboy]

Basic question, so apologies, but on my road-bike the brake blocks have worn down through the pre-moulded grooves so the contact area of the pad is now solid against the rim.

This is about 1/2 the total thickness of the entire pad, so could I just re-groove the pads with a sharp knife and get another 50% wear from them?

I've always replaced them at this stage before, but maybe that was too early?

They are replaceable cartridge type pads if that makes any difference? Maybe less likely to have a metal fixing post that could cause damage to the rim at this stage of wear?

[Rolls]

Replace imo

[okgo]

I replace when I hear metal on metal and not a moment before.