Okihara,
Your proposed method of charging lacked a cost "admin charge" for doing the calculations for the invoice to give to you.
This while you might think simple would actually take time to work out, confirm, enter and review. (All parts of it must be individually audited and costed to be in a positive income field and on a daily basis in most cases).
The current systems that flying schools have, are based more on a average based of yearly costs. So on average your flying cost of training will equal the same air time as others in the year you are being trained and the school will make an average profit based on hours charged on a meter.
So if you wanted a more detailed invoice of charges.
a) The hr rate will increase.
b) The admin fee will be introduced.
c) Contingency costs will probably be introduced to share the holding cost risks.
So it would in my opinion cost you more to do your training.
Any Jabiru engine done TBO without a Top? (honest question, think I only heard of one doing TBO with head work done).
I think CAsA do have a set of limits for chock to chock and wheels off to wheels on differences.
Now as for what you record for MR time, if a watch can be used - work out if you need a fast one or a slow one! never heard of CAsA inspectors requesting a calibration cert for a wrist watch yet! - no doubt this will be a Monday ramp check item.