....Or previous employer paid her a fair wage based on her skills.

OP don't let yourself get confused by the different factors. You also shouldn't listen to some of the crazy inflated rates that you hear on this board. Trust me paying more does not get you a better nanny.

1. Identify the non-negotiables legal, driving, how much experience etc
2. Identify fit factors or desirable qualities - high energy person who wants to go to many activities or do have lots of things to do around your house and want someone who stays in more, do you need flexibility in time or is your schedule predictable, do you have any parenting philosophies that are really important to you (CIO, AP, health foods, no TV, outdoor time everyday, want your child to be exposed to another language whatever), and other desirable qualities understanding that as you interview nannies you may or may not change your mind
3. Budget for around $12-$15 an hour average per 50 hours and factor in an extra 10% for taxes, workers comp etc. Negotiate in terms of weekly gross/hours and in your contract specify the base rate and OT rate. There are nanny calculators that will give you the base/OT rate from your weekly gross/hours.
4. Think through your compensation package but don't put everything on the table. Nannies are more focused on weekly pay so loading on all the extras doesn't make too much of a difference and doesn't leave you with anything to add down the road.
5. For PTO, its common to offer one week at the employer's choosing and one week at the nanny's chosen, a few sick/PTO days and a list of paid major holidays.
6. Negotiate with candidates as everyone will ask for a few dollars than their actual rate. In your reference check, check the previous compensation package to understand whether you nanny will have expectations that she didn't raise in the interview (previous employer let her take leave without pay for a month).

It all depends on what kind of nanny you want. Some will do nothing until you tell her, what and how to do everything, every step of the way. Some parents want the sitter to follow extensive instructions every single day.

Personally, I am not that type of nanny. Once I get the lay of the land, I know what I'm doing. I schedule routine parent meetings to make sure we're on the same page. Been doing this forever, so I've had extensive training and experience. And yes, I still love helping parents with their children.

BTW, the "I don't know what to do until you tell me" nanny, is significantly less pricey than the "she knows how to do her job" nanny. Of course, most parents fantasize about some happy mix of the two, in one person. "Obey my demands when I give them to you, but when I'm too busy, I want you to know what to do." Hence, the endless discord that we see on this nanny forum.

I should add, that often parents start out thinking they know which type of nanny they want. When it doesn't go so well, it's extremely difficult, if not impossible to fix. And there begins the nanny revolving door.

I hope you have lots of clarity and time to make the right choice for you and your children.

OP here -- forgot to say that we'd offer 3 weeks paid vacation (2 at her discretion, one at ours), all federal holidays, and the job would be for 50 hours a week (we'd pay overtime rates).

$12-$14 an hour. Rates are much lower in Silver Spring outside the beltway for a 2 kid job.

What would be a fair rate for a nanny in Silver Spring (just outside the Beltway)? It would be for 1 kid initially (around 17 months when it starts) and then for another kid starting in January (mom would be home with the new baby from October through January). Nanny would need to be able to drive as there's no close metro and we'll likely enroll the 1st kid in a morning school program in November.

Help! There are so many factors here -- new kid, half day school for first kid, etc -- that I'm having trouble coming up with a good rate.