I know this has been covered in past posts, but I guess I would like smart peoples’ take on a best next step.

I am considering new mechanicals in my large old (late-19th-c.) house and thought it would be smart to do a Manual J. I have done a fair bit of insulating and air sealing and am hopefully in a different place than when the previous owners installed their Big Ass Furnace.

Anyway, my question: after checking and double-checking, that infiltration HTM is clearly doing a LOT of work. And it’s based on scenarios that probably don’t apply to me. I’d like a more accurate number. From past posts, it sounds like my options are:

-Get a blower door test and adjust by another factor, which itself sounds fuzzy. (And given that I am nowhere near passive-house tightness, the error will matter more.)

-Do a workaround, such as an existing energy usage calculator somebody linked to at Borst Engineering

Is there a third option I’m missing? How would folks recommend proceeding? 

(Sidebar: at the risk of opening this question up too much, do the downsides of oversizing decrease with variable-drive mechanicals?)

PH