Oh that's a lovely lie in gb18!! Send some of that my way!
I would cap that 1st nap to 30 mins then try a 4hr 15mins A. Mine can quite happily do 4.5hrs on 30mins, but she might be an anomaly! (That was when I was trying the short am and long pm and it didn't work for us as she only napped 45 mins for the second one then was OT by BT) I rectified with a short 2.5-2.75 A before BT.
Could always try 4hrs and see how you go? How does your LO cope with OT? Mine has never done a 30 mins OT nap so I might not be the best one to give you advice.
Unfortunately for us DD napped in the car on the way home at 5.30 so is busy in her cot singing away to herself