How do i make a "fuel" and "refill" system?

In short i have a part with a number value named "Time" and has its number set to 100 and a surface gui with a text label inside of the surface gui. There is a script inside the text label that is supposed to tell the text label to set its label as the number value minus 1 every second to make it count down from 100. There is a button beside the part that adds 10 points to the number value on click. The maximum is set as 100 and if the player tries to spam the button, it will be capped at 100 and wont go any higher. My problem is when i try to set a minimum number value at 0 (so the numbers will stay at 0 and wont go any lower) it wont work. It looks fairly confusing to begin with as I've not done something like this before (or have really ever used values in a script). It's supposed to be a fuel bar and the button refills it just for context. Here's the script starting from the button to the main countdown script.

script.Parent.ClickDetector.MouseClick:Connect(function()
	if game.Workspace.CountdownPart.Time.Value <= 100 then
	game.Workspace.CountdownPart.Time.Value = game.Workspace.CountdownPart.Time.Value + 10
	end
end)

And here's the countdown script.

local countdown = script.Parent.Parent.Parent.Time

script.Parent.Text = countdown.Value

for i = 1,countdown.Value do
 wait(3)
 countdown.Value = countdown.Value - 10
 script.Parent.Text = countdown.Value
end

while true do
if countdown.Value >= 100 then
	countdown.Value = 100
end
if countdown.Value <= 0 then
	countdown.Value = 0
end
end