Personally, I tend to fall back on older edition rules mixed with current edition...
Less than 5 x Strength score = Unencumbered, no effect on movement. This would be less than 40 lbs. for the STR 8 character
5 x Strength score = encumbered, movement is reduced by 10 ft. (as per normal rules). This would 40 - 79 lbs. for the STR 8 character
10 x Strength score = heavily encumbered, movement is reduced by 20 ft. and disadvantage on ability checks, attacks and saving throws. This would be 80 - 119 lbs. for the STR 8 character. Personally, at this level I'd only let the character glide and not ascend
15 x Strength score = max load, movement is 5 ft., and disadvantage on ability checks, attacks and saving throws. This would be 120 - 320 lbs. for the STR 8 character; Personally, I'd rule this as slowed descent only (30 ft./round minimum).
30 x Strength score = max lift. For STR 8, this is 320 lbs.