I'm not sure about that, there was no 60 IG drop tank - 45 and 90 were standard sizes.
What it might imply is that internal fuel was not enough for anything resembling long range flight that might include combat. It took more than 25 IG to warm up, take off and climb to 25000 feet. 15 min of combat can use easy 25 gals - meaning that there is less than 35 IG fuel left to retrun to base. Those 35 IG 'buy' you 200+ miles in a Spitfire on relaxed cruise (can be lethal if there is LW to exploit that), less on faster cruise, and no reserves.
This is why people (including me) are suggesting the ~30 gal rear tank. Use that fuel until you're at 20000+ ft and above Channel, switch to drop tank, then drop the tank prior combat. After 15 min worth of combat, there is still 55-60 gals of fuel left in internal tanks to return to base.