Using Silicon / pH up late in flower?

[dingo bingo]

I'm using GA's coco nutes, currrently in week 6 of flower. I'm amazed at how much the pH drops with these nutes! My tapwater is 7.3 but last night I made up a feed (Bloom 4ml/L, XT 1ml/L, RT 2ml/L, BB+ 2ml/L) and afterwards the pH had dropped to 5.3!

 

I had no pH up so used Shogun's silicon (1ml/L only raised the pH up to 5.9) - they state it can be used late in flower but I'm sceptical as when I was growing 5 years ago I used PM's Bio-Silicon and that had to be stopped in week 2 of flower.

 

So, could anyone please advise if it's fine to keep using the Silicon or should I go and buy some pH up - if so is there an ideal type? (Eg. I understand phosphoric based pH down is better/more stable).