Both Severance Pay and Long Service Payment use the same underlying formula: two-thirds of your last month's wages, multiplied by your years of service, with the wage figure capped at HK$22,500/month and the whole payment capped at HK$390,000 no matter how long you've worked or how high your real salary is.
The 1 May 2025 abolition changes what happens next, not the formula itself. Your entitlement now splits into two portions around that date: a pre-transition portion, calculated on your wages just before 1 May 2025, which employers can still offset against accrued mandatory MPF contributions; and a post-transition portion, calculated on your final wages at termination, which must be paid out in cash in full, with no offsetting allowed at all.
That split matters most for anyone with long tenure spanning the transition date: the longer your service straddles 1 May 2025, the larger your post-transition portion grows relative to your total years, and the more of your final payout an employer can no longer chip away at with MPF offsetting.