The first step of casting a spell is to move it to the stack. If this causes the total toughness of creatures you control to change (perhaps because you control a creature whose toughness is determined by the number of cards in your hand), that new toughness will be used to determine the cost reduction.

Once you determine the cost to cast The Pride of Hull Clade, you may activate mana abilities to pay that cost. If the total toughness of creatures you control changes while activating mana abilities, the cost to cast The Pride of Hull Clade remains what you previously determined.

The Pride of Hull Clade's mana value doesn't change no matter what the total toughness of creatures you control is.

The Pride of Hull Clade's cost reduction ability can't reduce the total cost to cast the spell below {G}.

Once you announce you're casting a spell, no player may take actions until the spell has been paid for. Notably, opponents can't try to reduce the total toughness of creatures you control.

If the target of The Pride of Hull Clade's last ability deals combat damage to a player but leaves the battlefield before the granted triggered ability resolves, use its toughness as it last existed on the battlefield to determine how many cards to draw.