Plan a counter offer amount.
A counter offer should be anchored on the market rate rather than on what you currently earn, otherwise a low current salary permanently caps you. This anchors on the higher of the market median and the offer on the table, then adds negotiating room so that meeting in the middle still lands at or above market. The offer-versus-market figure tells you how much justification you actually have.
Anchor
target = max(market median, offer)
Counter
counter = target x (1 + negotiating room %)
Offer position
(offer - market) / market x 100
Because it anchors your future pay to a past decision made with less information. Employers pay for the role and the market, so argue from those.
Around 5-10% above your genuine target is normal and expected. Much more risks looking uninformed, especially when the offer is already above market.