What would be the correct serpentine belt size?

So I recently just added a smaller pulley and I’m waiting for my 90 mm aux idler to come in, the stock belt fits right now but I don’t think it’ll fit once the idler comes in what size would be the correct size to add?

I’m running a 2.76 smaller pulley with a 90mm aux idler pulley and the rest of the pulleys are stock

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2003RedfireVert said:

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https://lmr.com/products/2003-2004-Ford-SVT-Mustang-Cobra-Supercharger-Belt-Guide

Click to expand...

So from my understanding of the chart I will need to get a 74inch or a 74.63? Hopefully I’m reading it right

Stock lower pulley, 2.76 upper = 73.5

Add a snub idler (assuming a 90mm) = add 1inch

74.5in length which means the closest one is the 74.63in belt.

You are reading the chart correctly.

Here’s the Metco chart as well.