Plus it's high school not a 5 year tenure at 'Uni.
^ This... to paraphrase Winston Churchill... high school is not the end these days... it is not even the beginning of the end... it is perhaps only the end of the beginning.
Frankly IMHO High School graduation is to be expected, and a gift is appropriate, but a modest one...
I would also base the size of the gift on the level of achievement and effort shown... valedictorian with early acceptance to a 4 year university in an approved field of study would earn more of a gift from me than middling "Cs" and no clear path forward...
Large gifts would be reserved for proven performance beyond high school... finishing undergrad... larger gift... cum laude... magna... summa? Larger gift still... Master's Degree? Larger still...
Graduate Medical School or Law School? Largest gift... perhaps even paying off student loans if they were going into an approved field like concierge medicine or elective plastic surgery...
Anyway, long story short... high school graduation with no proven track record of success, or a mediocre GPA... maybe $500. You could even base it on performance... $200 per GPA point... up to a maximum of $800 for a 4.0... that way they have skin in the came and the harder they work, the greater the reward... just like the real world.
Now, for unsolicited advice... You have two children... you are leaving one family line 45% and the other 55%... the child without children's line is getting 10% less... in my experience this is a recipe for hard feelings, even if they kids won't admit it while you are alive... if you want to cause family angst for years after your death, favor one child's line over another's, or leave unclear instructions. If you left each child 50% would not the granddaughter's parents take care of her... "trickle down" style? It is your money, you do what you want with it, I only point out a potential pitfall.