I'm just in the process of building up a file for the Sakura Advance 2k18 Evo. Nearly done!!

Just got a question about springs. According to RC3's built in calculator the springs should be c=2.1
But if I measure the force required to compress the spring a given distance, it works out at c=2.7

Which method is going to be more accurate?

(Fyi spring specs are 1.4mm wire, 14 mm ID, 5.25 turns, 20.5mm long)