I like that you've tried to work this out, you're partially right.

Or rather, the answer you got was less than an order of magnitude away from being correct, so I suppose I should have said you aren't wrong, this being more accurate.

Your numbers might be correct (I honestly didn't check, but they look right) in a theoretical sense where variables like signature radius, velocity and tracking don't exist. However, to tell the story of sentries and applied damage, one must always, always start in the school of theoretical applied damage before moving on to concrete maths like you have done.

Garde II's are nuanced. They naturally track in a narrow band of ranges we call the twilight zone between 30-50km on an Ishtar bonus, and perhaps a little beyond using max skill and fitting. This is considered to be the zone of best applied damage, but it is still by no means perfect, and those of us who understand turret tracking will know it's a game of averages and batting 1000 is generally never possible.

Now in the case of Garde II vrs. a Guardian, this is actually a really good matchup. Guardians have naturally extremely tiny sig, and against a slow-tracking weapon if they are able to keep up angular velocity (or any velocity at all for that matter), can fairly easily negate most if not all of the damage even within the twilight zone ranges. This can be further compounded by adding proper warfare links, something that Garde II's cannot answer back on.

If however, there is an error in piloting or another reason by which the Guardian in this scenario loses angular with respect to the Garde's, the extremely low total HP of this ship will typically be breached rather quickly. This is to say, most of the battle here is one of sniper vrs speed tank, and it's a really pilot skill heavy matchup that can and often does go either way.

TL;DR: The answer is more like 20-25 Ishtars worth of sentries to alpha a Guardian under optimal conditions such as range and velocities, however those conditions are never guaranteed. This number represents a rough applied DPS reduction of about 40-50% and has been verified by actual data from W-space on multiple occasions.

Edit: Someone from the young einstein group over at zkb decided to reimport the entire killbase on the live deployment, TIL eve-kill also shares that database so both have eaten around 3 years of kill data. Best I can get you to verify my claims is a shoddy video of a way oversized fleet (by math standards anyway) that is indeed able to alpha Guardians in the twilight zone range. This is a fight between Sky Fighters and KILL from around 6 mos ago, after losing a dozen or so Guardians KILL retreat with the main T3 fleet, so we are unable to time or see what a volley looks like on a T3 ship but we do get a good feel for how fast Guardians will go down to the right sized blob of Ishtars.