You have to manage changing 3 parameters. Jump Strength, Gravity and Max fall Speed.
If you already done your work and want to change them to match the same platform but want to slow down the jump, you can use math to calculate you new speeds.
For example, If you already have these parameters as following:
Jump strength = 650
Gravity = 1500
Max fall Speed = 1000
And want to slow the jump into half of the original speed, you can re-calculate them as following:
Jump speed = 650/2 = 325
Gravity = 1500/(2^2) = 375
Max fall Speed = 1000/2 =500
If you want to reduce into the quarter of the original speed. It will be as following:
Jump speed = 650/4 = 162.5
Gravity = 1500/(4^2) = 93.75
Max fall Speed = 1000/3 =250
Hope this will help you.