RE: Depreciation calculation when Cost basis are changed in GP
As Shannon has alluded to, there is no simple formula to give you for this. I would argue that your scenario is anything but simple, to be honest, and to help identify if the calculations are correct or not would require more information - such as the values of the fields that Shannon mentions impact the amortization calculations. Your example has both a Cost change and a Useful Life change - which means you have 2 things impacting whatever choice you make in the Reset window.
When you add more cost to the asset, unless you change the placed in service date, the choice you make in Reset will make a big difference to your results.
Reset Life = as if the new settings were always in place from the beginning, regardless of when the additional cost was added to your asset. If you added cost and changed useful life to 5 years, it will assume the entire cost should have been amortized for 5 years starting from your placed in service date.
Reset Year = it will recalculate essentially from the start of your fiscal year onwards, similar to the above more or less, as if the new cost was in place at the beginning of the year + the 5 year life was at the beginning of the year, not in August when you changed it.
Based on what you are describing, it sounds like the proper choice in this case is Recalculate which then starts from this point onwards, in calculating depreciation but how that is done depends on depreciation method, averaging convention etc. If you're using Straight Line Original Life, adding life and cost might mean you want to change the method to Straight Line Remaining Life instead to get what you're trying to achieve - spread the cost/NBV out over the remaining life now that it's increased with a new component.
I hope this helps.
Jen