Pinging your mana should also show the actual amount
I had a game with Liss where I had like 140 something mana, which is just enough for a Q and W for reference. I pinged my jungler my mana bar to show I was low, apparently though, 140 mana is like 28% of my mana bar. He understandably got the wrong impression and ganked anyway while I had my metaphorical pants down getting ready to recall.
I'm pretty sure I'm not the only person this has happened to, so to maybe cut down on those rather unfortunate instances I think just having the amount of mana you actually have at the time could also be pinged alongside the percentage.
: 32% mana
An Ally has been slain
140 mana (14%)