LowEnergy is a small mod for Scrolling Combat Text that tells you (in yellow text) when your energy is < 15%.

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You can easily edit this to display (for example) 10% instead of 15% by changing the Lua to:
if energy==0 then
msg = 'Energy < 10%'
elseif energy<11 then
msg = 'Energy < 10%'

Just change the 'elseif energy<*number' then' to the number that's 1 bigger. (example: the number 12 would be 13) Then, change the 'Energy < ' to the exact number you want.
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All credit goes towards Gello.