Smooth pawn speed changing from current speed to set speed

Hi there.
There is a mechanic for spacecraft: lets say Current Speed is 0 and Set Speed is 0. Then I want to increase speed to 20. I press button once, Set Speed is now 20 and Current Speed starting slighty increasing 20.
My blueprint is not working properly.

When I press the SpeedUp button once to increase the Set speed, the Set Speed changes to 20, but the Current speed increases up to 23 instead of 20. If I press the Speed Down button once to decrease the Set Speed, the Current Speed changes dramatically.

What am I doing wrong?