When I think of deep stacks, I think of starting with 200-300 x the BB on the first level. Now to figure out your first level, you should decide what your "base chip" should be for the tournament. Tournaments are generally run using whole numbers so possible candidates for a base chip would be T1, T5, T25, T100, or T500.
T1: Not very common in mainstream, but can be used in home games, often because the host has a pre-denominated set that starts on 1.
T5: This was common in online games and can surely transition to live play
T25: This was the common base for the World Series of Poker main event (until sometime during the 2010s) when the starting stacks were T10,000 in chips and blinds starting at 25-50 (note that is 200 * BB in the first level) Most casinos emulated this for their own tournaments of all buy ins.
T100: This is now the common base for the World Series of Poker main event, blinds are starting at 100-100 with a starting stack of T30,000
T500: This is my personal favorite base for home games now. I like it because the next chip up from this is the T1000, so I don't put a ton of T500 in play which makes the first color up (the one involving the most players) easy.
The next chips up from these are typically the T1000, T5000, T25000, T100000, and possibly T500000 however they would be functionally the same as the T1, T5, T25, T100, and T500 as "base chips" so they don't merit much consideration.
So what you do is decide on your base chip, and include that plus the next 4 (5 at the most) denominations.
Then you design your starting stacks, multiply your starting stack by the number of players you expect, and add larger chips on to account for color ups and re-entries.
Possible 200BB starting stacks for each base chip
| Base | First blind level (1 chip-2 chip) | Denominations | Example Starting Stack Quantities |
| T1 | 1-2 | T1/5/25/100 | 15/17/8/1 or 10/12/5/2 (total T400) |
| T5 | 5-10 | T5/25/100/500 | 15/17/5/2 or 10/10/7/2 (total T2000) |
| T25 | 25-50 | T25/100/500/1000 | 12/12/5/6 or 8/8/4/7 (total T10000) |
| T100 | 100-200 | T100/500/1000/5000 | 15/5/12/5 or 10/6/7/6 (total T40000) |
| T500 | 500-1000 | T500/1000/5000/25000 | 6/12/12/1 or 4/8/8/2 (total T100000) |
Decide which of those look "good" to you and go from there.
So for example, say you pick T25. You know your starting stacks need to be at least 8/8/4/7 * 18 players means you need 144/144/72/126 of T25/100/500/1000 in starting stack chips, plus it's a good idea to add enough rebuy chips for at least one per player plus color ups, so call that another 40. Round your quantities up to the nearest 20 and you get a buy of 160/160/80/140/40 or 580 chips. If the purchase requirement is in multiples of 25 then the buy would be 150/150/75/150/50 for 575 chips. You of course can tweak these to whatever figures fill out your case or what have you.
If you prefer putting more chips in the starting stack to reduce changemaking, you can copy the process above using the 12/12/5/6 format.
You would need 216/216/108/112 for your starting stacks. Then still add 40-50 more T5000s for re-entries and color ups. That would give you a buy of 220/220/120/120/60 for 740 chips, or if buying in quantities of 25 225/225/125/125/50 for 750 chips.
Hope this helps.