Second UK visa refusal under V4.2(a) and (b); what can I change? [duplicate]

My visa application to visit for training and taking exams has been refused twice, on grounds that the cost of my trip represents three times more my stated disposable income. This is the information U had provided on the application form:

Salary: £964
Monthly expenditures: £535 (I actually don't spend it all, unless it's an emergency)
Disposable income: £428 as per the ECO calculation after deducting my expenses from my income
Stated trip cost: £1500

However, I wrote £535 as a rough estimate. I send money home to my family sometimes, and I normally save all my salary. I earn extra income from tips and allowances, and receive a food allowance, too, which I did mention. I don't think that this was considered, although it's evident on my pay slips which show £200 food allowance and service charge, and £220 per week in tips.

I want to visit for 4 days in July to attend a training, and 2 days in Septmeber to sit an exam. The cost of the trip came to be £1500 and includes flights, hotel, and food. I have already paid in advance for flights, accommodation, and training fees, and I attached the receipts.

1. In my next application, should I reduce my monthly expenditures?
2. Should I reduce the cost of the trip to only food and ground transportation, and state that the flights and hotel are paid for, attaching receipts as evidence?
3. Will this be enough proof to show that I am not planning to spend more than I can afford?

Note that I have £4500 accessible through my debit card and £4000 available on my credit card. I can obviously afford this trip but how can I correct how I appear financially?