When should the 800,000 baht be deposited for a Thailand retirement visa extension?

My question is regarding the 800,000 for retirement extn. I have used the regular embassy letter for about 8 yrs. I have differing opinions re. how early this 800,000b should be placed in the bank. One opinion is 3 months before the due visit. Another guy told me that 3 months is first-timers only and that old hands can place in the bank 2 months before. Further question: my bank is SCB in city centre (near my old job). I believe I have to show a letter certifying that they have my money. Do I need to get the letter from the actual branch? (I don't live there now). My friend banks with BKK Bank far from CW but he pops downstairs and gets his letter at CW the same day (even though it costs 200b). Also--Can I keep the 800,000 there for 12 months, or must I spend 400,000 after 4 months. I'm confused. Thx.