Chain length with 34 T rear sprocket and triple front

I've just changed over my drivetrain (put the old parts back onto the old bike) with an SLX triple and XT 11-34T cassette. Rear shifter is an X9 long cage FWIW. I'm just wondering what the accepted wisdom is with chain length and the bigger big rear ring... I've played safe for now and left it fairly long, so that it'll shift into big/big if I should stupidly do that while riding... Obviously I shouldn't ever be in the crossover gear, but sometimes you make mistakes. (though this would be a daft one to make)

But, it's pretty slappy in the lower gears. So, I'm wondering if it'd be better to lose another link, and just not screw up? What's the standard position with these?