No. When you've lost half your health, you have a 5 point reduction shield. At 10hp, you have a 9 reduction shield, a shield that can nearly block dragons. All for the cost of 6|5 
? OP. Sure, it's pretty useless early on, but as you take damage, (and you'll take less and less), the shield gets more and more powerful.
RoL/Hope has the same [much better] effect without the risk of non-physical damage or minor high-hitting creatures (albeit for a 3-card combo, but this requires only one element), as well as not running the risk of PC. Dissipation field is also useless early on but gets more powerful later, for less quanta and more blocking, but is not regarded as OP. Gravity shield can block most dragons, but it's regarded as useless.
I like the card; it seems roughly balanced (perhaps more on the side of powerful).
RoL/Hope, while potentially better, is also quite shaky due to needing a 3 card combo, and at least 9
as well as 7
to set up. Not to mention it's quite weak to CC. EtG has way more CC than PC cards. Dissipation shield is not OP because it requires the user to sacrifice precious quanta. Quanta is especially important early on in the game, and even as the game drags on, it can be worn away by creatures. Compression barrier is a 5
card that could become an insanely powerful shield with only momentum, spells, or PC being the way around it.
Changing the block rate to hp lost/20 would make it not as powerful and probably quite weak.
