Firstly ... Is the offer reasonable? Is it consistent with reasonable selling prices of that same model and year in similar condition?
If "yes" then just because someone says "not to bite on their first offer" doesn't mean you should follow that advice. If the offer is reasonable, and would allow you to purchase the same model and year in similar condition including the taxes, then it is likely not worth your time and effort to argue about it. Accept it and move on with life.
If "no" - the offer is unreasonably low - then do some research. Go through classified ads. Put together a list of what YOU think are comparable for-sale ads and present that to the adjuster. Don't forget that the amount they pay you also includes the taxes, and the classified ads won't. But also don't forget that in classified advertisements, the "asking" price is one thing, and the "selling" price is quite another. Don't think for a moment that the insurance company doesn't know this. Don't cherry-pick your advertisements when doing this. Reject the ads where someone is dreaming in technicolor about what they think the bike is worth. You wouldn't buy that; nor is the insurance company obligated to pay you that much.
If the offer wasn't reasonable then I would have a simple thing to say to the adjuster: "You show me where you can buy a 20XX model Z with under YYYYY km in good condition for that price."