People are either bottom-up thinkers or top-down thinkers. I can tell from your description that you are a bottom-up thinker, more comfortable finding details first and assembling the big picture out of a sea of data. I am, too, and most engineers are. Top-down thinkers - perhaps like your husband? - often start with a vague idea of the direction they want to go, perhaps write out an outline, and gradually fill in the details.
The good news is, you can solve a problem either way. So, let's attack your problem from the alien, top-down method.
Figure 1.6% WR a year, and divide that by 12 for a monthly budget. Could you live on that in a typical month? Does it pass the gut check? Remember that you'll need to pay taxes on this, so fill out a 1040 with some estimated dollars using that 1.6% figure. Maybe throw in some estimates for part time work at your current business.
As a bottom up thinker, this exercise will drive you toward filling in more of the details. You'll start building better models and getting more specific about your expenses. You can model what your life would look like on 150k, 120k, and 90k. Incidentally, look up Root of Good's sticky post in the tax forum where he shows how he made 6 figures and paid no taxes. Really inspiring example, but very specific to his situation. There's stuff to learn there, though.
Regarding the method of retiring, most successful ER's seem to ease into retirement, taking some part time work or other tactics to reduce their stress and have more time for family and fun. Especially when a business ownership stake is involved, selling it before you're comfortable with retirement is one way ticket. Best to keep a stake in it for at least a year and see how you like retirement.