FICO scores revolving CCs very easily by considering the balance and the CL. Open charge accounts like Amex don't have a reported CL. Because they don't report a CL, they are factored differently into your FICO score:
For EQ (and EX) FICO scores, the balance and the reported high limit are not factored into your FICO score at all. They are ignored. It doesn't matter if you have a reported balance of $5 or $15,000, the impact is equal in that FICO ignores it. The only thing that your EX and EQ FICO scores would consider is the account type and the length of history. The balance and reported high limits are ignored by FICO.
Now there are different versions out there of your FICO score. Newer versions of your TU FICO are scored just like in the above for EQ and EX. However, myFICO.com and many lenders still use an older version of TU FICO (called TU98). This version scores your Amex very differently. This version of TU FICO factors in both your balance and your "high balance". The high balance is scored as the CL by the FICO score. So, if you have a $1000 balance on a $5000 high balance, then your utilization for this under a TU98 FICO score would be 20%. As mentioned, newer versions of your TU FICO would not score it as 20%...because the balance and high balance are ignored in scoring it.
As an example, if you have 3 CCs, one being an Amex Green (or Gold, or Platinum, or whatever) with a balance of $1000 and a "high balance" of $5000, and you have two revolving (Visa, MC, etc.) of $750 each at a CL of $1000 each, then under the same version of FICO score as pulled from here for TU, you have $1000+$750+$750, or $2500 in balances with $5000+$1000+$1000 or $7000 in CLs, or 35.7% utilization. If you have that on your TU FICO report, you'd see a figure of 35.7% utilization. Why, because both the balance and the "high balance" are factored in.
However, if using the same above for considering utilization on your EQ report, your utilization would be 75%. Why? Because under EQ (and EX), the balance and the high balance are ignored. Because it is ignored, only two cards are scored per the utilization. The same applies for newer versions of your TU FICO as used by some lenders. If they factored in the above using these numbers, they'd also see 75% util. Now other factors continue to help like the history or mix of credit.
Per your mix of credit (10% of your score I think), you should have a solid mix of both revolving and charge cards for optimal points. If tthe Amex is your only card, then consider adding at least a revolving. Now you can certainly hit close to 800 with just an Amex. Depends on your goals.
