I've shot lead through Glocks that barely lead at all, and I've shot some that leaded terribly.
And that's the problem in a nutshell. There is no good rule of thumb on how to be sure how to be safe in terms of keeping the gun from leading to the point that it becomes a problem. What works well in one Glock might lead dangerously in another apparently identical pistol.
At any rate, if you do shoot lead through a Glock (understanding that Glock and other experts on the topic strongly recommend against it) you should be aware that you should NEVER run a jacketed or plated bullet down the bore after shooting any lead until after the bore has been thoroughly cleaned and any/all leading has been completely removed.
It would also be very wise not to push the envelope at all in terms of trying for maximum velocity since you're already essentially ignoring one known risk factor.