I'm no expert. I've just been through the wringer with an ex who let his g/f run the show - still does. Keep precise records as you never know in the future when you will have to go back over everything.
Budget is very, very important as it sets the stage for what's to come in the future. Once your house sells there may or may not be an adjustment - that will depend upon what you can hopefully negotiate. If not then it comes down to a judge's discretion. Judges discretion can be a good or a bad thing - kind of like throwing the dice.
You should honestly list what your normal expenses on a monthly basis. So you don't get too overwhelmed I'd recommend putting things in categories. Personal - hair, nails, clothing (I had a difficult time with this as I have never been much of a shopper unless for golf stuff. Problem was that one does have to buy clothing, shoes, coats from time to time). In another pile you can label it as health. This would include prescriptions, chiropractor, prescription glasses,etc. I would think this is important as I recall in a previous post you indicated that you had high medication cost. It wouldn't hurt to have your pharmacist give you the cost of your meds if you were to pay for them without a health plan coverage. Food - if you entertain family frequently, make sure you put in the cost. Gifts would be a category, particularly for children and grand children. Charitable donations. Entertainment can include taking your kids to a show, personal sports, etc. Travel. Vehicle maintenance and insurance. Taxes and debt. You will soon prepare your income tax return and will know what your position is with CRA.
After you have compiled all the separate piles you will probably be a tad overwhelmed. This is normal. Remember it is better at this point to overestimate rather than underestimate. It can be difficult to go back to court to get more money 12 months from now and convince a judge that you can't live on the budget you previously submitted. You then calculate the taxes that you estimate you will have to pay.
I would recommend that you keep the down payment and moving expenses for your new place separate from your personal budget.
Yes the budget is an integral part of the start of your negotiations for spousal support.
Always remember that it is a really good idea to read up as much as you can about spousal support. For your reading pleasure I will attach a recent decision that was just decided this month. Long term divorce (26 years) of two wealthy, successful doctors. Yes the wife, who is a successful OBGYN, was most definitely awarded spousal support. What I wouldn't like to be receiving her monthly cheque! And yes she would have had to have put together a budget at some point during her litigation. In this case the budget would have been for her medical practice.
https://www.canlii.org/en/pe/pesctd/doc/2015/2015pesc1/2015pesc1.html
It's an application with no court date yet and has 30 days to respond once served which should be very soon. Said we can then proceed with scheduling a case conference or bringing emergency motions as may be necessary, if he cuts benefits, and restraining order as well. So case conference? what to expect and next?