A system comprising: at least one computing device configured to tune a gas turbine out of a set of gas turbines (GTs) by performing actions including: commanding said GT of the set of GTs to a base load level, based upon a measured ambient condition for said GT; commanding said GT of the set of GTs to adjust a respective power output to match a scaled power output value equal to a fraction of a difference between the respective power output and a nominal power output value, and measuring an actual emissions value for said GT during the adjusting of the respective power output; characterised by adjusting an operating condition of said GT of the set of GTs based upon a difference between the respective measured actual emissions value, a nominal emissions value at the ambient condition and an emissions scale factor in a way to bring the GT's actual emissions value closer to the nominal emissions value, wherein the adjusting of the operation condition of said GT of the set of GTs by a fixed fraction of the difference between the respective power output value and a nominal power output value aligns said GT on a line (300, 302, 304, 306) in graphical space plotting power output versus emissions that is orthogonal to a nominal power output/nominal emissions characteristic for the said GT. at least one computing device configured to tune a gas turbine out of a set of gas turbines (GTs) by performing actions including: commanding said GT of the set of GTs to a base load level, based upon a measured ambient condition for said GT; commanding said GT of the set of GTs to adjust a respective power output to match a scaled power output value equal to a fraction of a difference between the respective power output and a nominal power output value, and measuring an actual emissions value for said GT during the adjusting of the respective power output; characterised by adjusting an operating condition of said GT of the set of GTs based upon a difference between the respective measured actual emissions value, a nominal emissions value at the ambient condition and an emissions scale factor in a way to bring the GT's actual emissions value closer to the nominal emissions value, commanding said GT of the set of GTs to a base load level, based upon a measured ambient condition for said GT; commanding said GT of the set of GTs to adjust a respective power output to match a scaled power output value equal to a fraction of a difference between the respective power output and a nominal power output value, and measuring an actual emissions value for said GT during the adjusting of the respective power output; characterised by adjusting an operating condition of said GT of the set of GTs based upon a difference between the respective measured actual emissions value, a nominal emissions value at the ambient condition and an emissions scale factor in a way to bring the GT's actual emissions value closer to the nominal emissions value, wherein the adjusting of the operation condition of said GT of the set of GTs by a fixed fraction of the difference between the respective power output value and a nominal power output value aligns said GT on a line (300, 302, 304, 306) in graphical space plotting power output versus emissions that is orthogonal to a nominal power output/nominal emissions characteristic for the said GT.