The law's specific on this, you get the protection for the whole cost of an item or service, even if you only pay for a part of it - even just 1p would count - on credit. The only condition is that what you're buying costs more than £100 and less than £30,000.
Therefore if you want protection…
As long as it costs more than £100, pay for even a fraction on a credit card and you're protected.
Here's a Section 75 deposit-only success story to give you some inspiration...
I ordered and paid £15,991 in full for a new car but before I took delivery, the trader went into liquidation.
Thankfully I had paid the first £100 deposit on my Barclaycard credit card. So I made a Section 75 claim. It took six months, but this week I received a credit to my card of the whole amount, just from having paid the first £100 on my card.