I'm so fed up with T&M that I try my hardest to either bid, or quote a job. And for residential service work I'm going to flat rate. Here's why;
First and foremost these items always happen AFTER most or all of the job is done! Now you have to fight to get paid!
The bigger the job is the more you are scrutinized for everything! You will have to defend your every movement. I can't stress this enough!
You end up getting accused of going slow on purpose.
They want you to justify every parts cost, and will not pay for those left overs. You get to go and return them on your own time!
You end up fighting over time doing things like layout, getting parts, and how long it all takes.
They call around to compare you to others, and always find someone who says he could have done it in less time.
They don't want to pay for, going to the bathroom, answering the phone, a quick coffee, reading the instructions, etc.
Also, It guarantees that you will not make any profit. You are punished for having all the best tools and training, and are quick, because instead of being rewarded for working really hard and fast you get paid less. How is that fair?
If a device you provided fails you still have to come back and fix it for no extra money. Your markup is supposed to be your insurance policy to pay you for those times it happens.
Need I go on?
Rick