i've been thinking about this exact question lately, so....
1) i assume this is for u.s.-friendly sites.
2) i assume that you want to minimize transfer fees and times both for e-wallet purposes and rakeback purposes. this matters for epass, full tilt, and cake (as far as i know).
3) i assume that you don't want to play underrolled for a given site or game (i.e., you have to get up from a game due to insufficient funds to reload).
4) i assume that once you pull money out of the checking account, it has officially left the poker world. that is, you (basically) have an irreplaceable roll, and you don't want to or intend to redeposit.
5) for lower variance games (e.g., ones in which you have a lot of calling stations or weak-tight players), i think you need a number of BIs that is equivalent to the number of tables you play plus five. for example, if you normally play six tables, you would want 11 BIs for that site.
6) for games in which you're getting in in with a lot of marginal hands, then i would bump the reserve to eight BIs. so if you play six tables, have 14 BIs at a site.
7) if you play a range of table stakes (e.g., due to table selection, game availability, etc.), use averages. for example, if you usually play have 600nlhe and half 1000nlhe, consider one BI = 800.
8) keep a reserve amount equal to an equivalent of one "reserve" in a wallet (e.g., 5 BIs or 8 BIs).
9) have a "reserve roll" of overage in the high-yield checking account (for an irreplaceable roll, i'd save up until i could match what i had on the sites and e-wallets)
10) take the rest off the site and spend/invest (i recommend mostly the latter).
11) for sites that have no fee penalties and process relatively quickly, then i might lower these requirements. for example, it's relatively easy to get money in and out of stars, so i'd have no more than tables+five and possibly as low as tables+3 (depends on games).
12) withdraw the overage from sites in set increments (e.g., $1k or $2k -- whatever works for you).
this may seem REALLY conservative, but the rationale is this. you want to be able to sit at the tables full-stacked (or whatever stack you usually have) AND be able to reload to at least your "regular" stop-loss limit (most of the greats do it, so you can, too). if you're stop-loss is 4 BIs, then you will still have a full amount on the site to buy-in AND still have reserves for your next session. the goal is never to have to avoid sitting in a juicy game due to insufficient funds.
once you take a big hit, start reloading the funds from the e-wallet asap -- at $500 or $1000 a pop takes some time to reload $4k. likewise, if your online roll doesn't recover relatively quickly, start pumping money into your e-wallet from the checking account. since there are fees involved in this, if you find yourself pushing money into and out of a site quite a bit, then i'd put more reserves in an e-wallet.
once reason i like this system is that it keeps me relatively hungry -- i never really seem flush with cash on a site, so i don't get careless. obviously i HATE to reload, so ymmv
fwiw, i have about 30 BIs across four sites and 5 BIs in an e-wallet. the rest is in a high-yield checking account (still saving for reserve roll). 6 of those BIs are at a skin and are purely for bonus purposes since the bonus is superior to rakeback at the other skin.
note: if you're doing some sort of kelly criterion shot-taking, then this whole plan flies out the door. in fact, to use that, i'd probably keep all of my active br on one site until i leveled off in stakes.