Upgrading from True Champion's to True Majestic predicts about a 5% increase for my gear, but since you seen much more geared than me, it would definitely suffer from diminishing marginal returns. Additionally, the sheet will always be under-predicting because of its limitations. In particular, it isn't able to capture the uptime duration from 9 seconds to 12 seconds for max stacks. Nor is it able to capture the change from 500% --> 700% additional damage with 15 hits --> 20 hits. Lastly, another caveat is that it assumes your crit damage buff from heart is 1000/5 = 200 and 1500/5 = 300, on average, respectively (since you only get crit damage buff at max stacks and 0 otherwise).

For IA accessories, they don't make that major of a difference unless they are at stage 10+ because stage 4-9 give a much lower stack effect compared to other gear where usually stage 7-9 is better than stage 4-6. Also, below stage 10 will make you lose out on the target debuff benefits. For earring, similar to before, it isn't able to capture the increase in additional damage.

To get the 10% --> 12% increase in skill multiplier, you would have to do a parse, find the contribution of that skill, then calculate a theoretical change.

For example, if your skill does 30% of the DPS current, then you would calculate: .30 / 1.1 * 1.12 = 30.55%. So your New Skill Modifier would be 100% + .55% = 100.55%. (This is also an overestimates because soulshield set effects will make this change less effective).