Flail

Type	Power	Accuracy	PP	Priority
Normal	-	100%	15	0

N = floor(64 * User's current HP / User's total HP)

Alternately, you could do it as a direct function of your % HP, although this value could be off on borderline cases due to rounding.

Pokémon	Tier	Abilities	HP	Atk	Def	SpA	SpD	Spe
Bellossom	UU	Chlorophyll	75	80	85	90	100	50
Blastoise	UU	Torrent	79	83	100	85	105	78
Dodrio	BL	Early Bird / Run Away	60	110	70	60	60	100
Donphan	OU	Sturdy	90	120	120	60	60	50
Dunsparce	UU	Serene Grace / Run Away	100	70	70	65	65	45
Espeon	BL	Synchronize	65	65	60	130	95	110
Farfetch'd	NU	Inner Focus / Keen Eye	52	65	55	58	62	60
Flareon	UU	Flash Fire	65	130	60	95	110	65
Forretress	BL	Sturdy	75	90	140	60	60	40
Gyarados	OU	Intimidate	95	125	79	60	100	81
Heracross	OU	Guts / Swarm	80	125	75	40	95	85
Jolteon	OU	Volt Absorb	65	65	60	110	95	130
Kabutops	UU	Battle Armor / Swift Swim	60	115	105	65	70	80
Kingdra	BL	Swift Swim	75	95	95	95	95	85
Kingler	UU	Hyper Cutter / Shell Armor	55	130	115	50	50	75
Lanturn	UU	Volt Absorb / Illuminate	125	58	58	76	76	67
Ludicolo	OU	Swift Swim / Rain Dish	80	70	70	90	100	70
Luvdisc	NU	Swift Swim	43	30	55	40	65	97
Meganium	UU	Overgrow	80	82	100	83	100	80
Milotic	OU	Marvel Scale	95	60	79	100	125	81
Ninetales	UU	Flash Fire	73	76	75	81	100	100
Parasect	NU	Effect Spore	60	95	80	60	80	30
Pinsir	UU	Hyper Cutter	65	125	100	55	70	85
Qwilfish	UU	Swift Swim / Poison Point	65	95	75	55	55	85
Sandslash	UU	Sand Veil	75	100	110	45	55	65
Seaking	NU	Swift Swim / Water Veil	80	92	65	65	80	68
Slaking	OU	Truant	150	160	100	95	65	100
Spinda	NU	Own Tempo	60	60	60	60	60	60
Steelix	BL	Sturdy / Rock Head	75	85	200	55	65	30
Sudowoodo	NU	Sturdy / Rock Head	70	100	115	30	65	30
Tangela	UU	Chlorophyll	65	55	115	100	40	60
Torkoal	NU	White Smoke	70	85	140	85	70	20
Umbreon	OU	Synchronize	95	65	110	60	130	65
Vaporeon	OU	Water Absorb	130	65	60	110	95	65
Vileplume	UU	Chlorophyll	75	80	85	100	90	50
Zangoose	BL	Immunity	73	115	60	60	60	90