Your sentence is perfectly good. I'm not sure which part you want to change. let me know ~ tell me, send me word, have John tell me [none is better than what you started with] when you've finished/done ~ you're through
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MaanuMp'Please let me know when you have finished/done'Have done is wrong. ... when you are/have finished or when you are done.
Cool BreezeMaanuMp'Please let me know when you have finished/done'
Have done is wrong. ... when you are/have finished or when you are done.
CB
Good catch, C.B. I missed that one.