How to increment this slider UI by 5, and then count how far it has been dragged?

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#1

I am making a slider bar for Mobile users that allow them to change the amount of power they can use based on how far the frame has been dragged.

local absoluteposition = Vector2.new(sliderUI.AbsolutePosition.X, sliderUI.AbsolutePosition.Y)
local absolutesize = Vector2.new(sliderUI.AbsoluteSize.X, sliderUI.AbsoluteSize.Y)
local Multiplier = absolutesize.Y / 15
repeat
			
			local sizeRequirement = (absolutesize.Y + absoluteposition.Y) - mouse.Y
			local completeCalculation = math.floor((sizeRequirement + (Multiplier / 2)) / Multiplier) * Multiplier
			
			local powercalculation = math.floor(completeCalculation / 5)
			print(powercalculation)
			
			if completeCalculation > 0 and completeCalculation < absolutesize.Y then
				slidebar.Size = UDim2.new(1, 0, 0, completeCalculation)
			end
			
			
			task.wait()
		until not holdingbar

The issue I am having here is that I am not able to figure out how to calculate the power when the UI is being dragged (The UI works perfectly fine)

Video of the UI: https://gyazo.com/ffe5743cf41f7531e5517f69af21400b

I tried calculating the power by dividing the pixels by 5 and rounding it, but then the result does not give me numbers that are divisible by 5 and sometimes adds 6 from the previous answer.
Example:
image

#2 
local oldNumber = 13
local newNumber = oldNumber - (oldNumber % 5)
print(newNumber) --10

Subtract the remainder of the number divided by five from the original number.