I've been holding them against, mostly too.
If you hold it above the card, both PH and Nitrate give a totally different reading (at least one or more steps lower).
Guess I've been doing it right, but when I saw the LFS
using the same test reagents, and standing them up on end, it made me wonder.
180G build thread
180G All-glass with dual megaflows in a custom stand
AquaC EV-180 skimmer
250 pounds LR
160 pounds aragonite
Custom acrylic sump from GlassCages.com
2xPCX-40 return pumps
2x250MH 14K, 2x175MH 14K - PFO lighting, 2x65W PC
No-name 36W UV
Water General 125GPH RO/DI