I'd recommend just putting off your initial dose for as long as you can manage, and in the interim simply getting yourself into the right headspace/set leading up to the drop. Perhaps hanging with others who've already taken something and running off their contact high?
As an example, when I go out clubbing I usually aim to drop between 10pm and 10:30pm...though the last time I was out I completely missed that timeframe and ended up dropping at about 11pm....why did this happen? Well I was in the perfect set and setting already, and honestly didn't even realise I was 100% sober for that extra hour or so LOL. Felt like a very nice buzz with heaps of increased sociability and empathy just from being in the right environment. As a result, my "roll" for the night was effectively extended by an hour or so depending on how you look at it, as I was already having a great time off the contact high before I'd even ingested ANYTHING except water that entire day lol
Not only is the above great HR advice, but I honestly believe it's the way to go regardless of what/how much you plan on taking through the night. Made my 200mg+100mg booster last all the way through until around 3-4am the next morning, and even then it was only ended by the fact my body was completely fucking exhausted, and my desire to sit down and rest ended up overcoming the euphoria lol
That being said, even though it's never a great idea regardless, you did claim this will be your last ever roll. I'm not going to recommend you take the entire amount you desire (600mg) as that's definitely not good HR....but I'm sure there are plenty of things you can do to minimise any repercussions should you decide to go through with it. Placebo/anecdotal or not, I'd recommend pre-loading 5HT-P leading up to the roll (not in the days before though), getting your hands on something like Vital Greens (greens supplement, IME this has greatly enhanced my rolls), the usual healthy eating and exercise before and after the roll, and post-loading 5-HTP in the days/week afterwards.