Was my mash pH okay? Should I be adjusting it?

I am still new to brewing: First attempt with Christmas gifts used a refill kit that turned out to be partial - resulted in stout that tasted fine but is very weak. Second attempt is due to be bottled later this morning. This second attempt is a "North of England" brown ale LME / grain kit.

I've been reading quite a bit from John Palmer's "How to Brew" which seems to be a pretty complete guide, and I'm reading a lot about water. I've managed to get the local water reports (Irving,TX - water supplied via Dallas). Using the calculators inside the back of the book, I'm coming up with a mash pH of 5.7-5.8. This seemed a good match for a brown ale.

I measured the mash with brewing pH strips (as I'm starting out, I'm wary of investing in too much expensive equipment upfront - I can get a pH meter later if necessary). Okay pH strips aren't the most accurate, but I came up with a result more in the 5 - possibly 5.4 range. This is way off the bottom of the scale in the book!

I guess my question is, is it possible the mash was really that low? Or are the papers really that inaccurate? Of course the mash was a brown colour (as were the indicator colours), and the papers have a 0.4 resolution in the scale indicator. Could mash temperature have influenced the reading?

If it helps, the kit was 50/50 Amber and Dark malts with speciality grains mostly Caramel (3/4th) and lesser (1/8ths) special B and chocolate.

Looking forwards, I plan to brew quite a few English porters (a type of beer I like), but will not start these until Autumn (however much I like a good porter, they're not for a Texas summer!). I'm assuming I should adjust the chemistry for these? Before that, I intend to brew paler beers - I've just ordered an ESB kit, and after that I was thinking of more of a summer pale ale.