The magic blocking works exactly like Physical damage block, so it'll take a huge chunk out of your stamina, even at DSS+5, since the largest amount of Guard Reduction that you'll get is +69%, which is not much, and yes, it's 100% magical damage reduction, you won't take damage from magical attacks, but you'll take damage from Fire Attacks, so be careful with what attack you block.
My suggestion is to upgrade your endurance to level 40 (since your stamina caps at 160, so even if you raise endurance to 99, you won't have more than 160 stamina) so that you'll be able a bigger amount of subsequent blocks. Also Eternal King Ring helps a ton, since your stamina will recharge at a much faster rate.
To me, the best shield in the game is the PFS+10. It has an insane amount of Guard Reduction (+83%) and good Fire Reduction, plus it's weight is the smallest for the same class of shields (8.0 lb).
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
